Nuclear Quadrupole Coupling in

Transition Metal Compounds

by

Shen-Dat Ing

'Ihesis submitted to the Graduate Faculty of the

Virginia Polytechnic Institute and State Unlversity

in partial fulfillment of the requirements for the degree of

APPROVF.01

Thomas C. Ward

H~r.old M. McNair

Doctor of Philosophy

in

Der.arlment of Chemistry

Ray" F, 1Tipswora' f November, 1971

B1acksburp,. Virgln1~

I > -

- "'

TABLE OF CONTENTS

ACKNOWIEDGEMENTS. • • • • • • • • • • • • • • • • • • • • • • •

LIST OF TABLES. t t t t t I I I t I I I I I I t I t I I e I I I

UST OF FIGURES •

INTRODUCTION. • •

• • • • ' . . . . . . • • • • • • • • • • • •

• • • • • • • • • • • • • • • • I I I I I f I

REVIEW OF THEORETICAL CONCEPTS

A). NQR Energy Levels and Transl tions. . . . ' . . . . B). Interpretation of Nuclear Quadrupole Coupling Data

1). Townes Dailey Empirical Approach ••••••• 2). Semiquantitative Quantum Mechanical Evaluation

UTERA TURE REVIEW

• •

• • • • • •

A). Bis(tetracarbonylcobalt)tin Derivatives. • • • • • • •

B). Copper(I)'Ihiourea and Substituted 1biourea ComplP.xes • 1). Thiourea Complexes ••••.•••••••• , • 2). Substituted 1biour.ea Complexes •••••••. ,

EXPERIMENTAL

A) • Preparatory Work 1). Bis(tetracarbonylcobalt)tin Compounds. . . . 2), Copper(!) Thiourea and Substituted Thiourea

Compounds. • • • • • • • • • • • , • • , , , • • •

Page

iv

v

vii

1

4

21 21 28

34 46 46 52

54 54

54 B). Instrumentation 59

1). Superregenerative Zeeman-Modulated Spectrometer. • 59 2), Method of Frequency Measurement. , ••••• , • • 60

C), NQ.R Data • • • • • • • • • • • • • • • • • • • • • • • 64

DISCUSSION

A). Bis(tetracarbonylcobalt)tin Compounds. . . . . . . ' . 76

ii

I

iii

Page

B), Copper(!) thiourea and substituted thlourea complexes ••• • • • • • • • 84

C). Molybdenum Oxyhalides. • • • • • • • • • 109

BIBLIOORAPHY. • • • • • • • • • • • 112

. . • • • • • . . • • • 115

ACKNOWLEDGEMENTS

'Ille author wishes to express his appreciation to his major

professor, Dr. Jack D, Graybeal, for his patience and help during the

courses of this investigation. He would also like to thank his parents

for their constant encouragements and sacrifice, without that the task

of this work would be impossible.

'Ille financial support of the National Science Foundation, as

well as the Chemistry Department at Virginia Polytechnic Institute and

State University are both acknowledged with gratitude.

iv

Ta.ble I

'!able II

Table III

Table IV

Table V

Table VT

Table VII

Table VIII

Table IX

nt.ble X

nt.ble XI

Th1'le XTT

Ta.l-~_e XIV

Table XV

Table XVI

nt.ble XVII

LIST OF TABLES

Secular Equations for Nuclei with Half Integral Spin, , • , , , • , , , , • , • • • •

Fol'llulas for the Nuclear Quadrupole Resonance Frequencies, , , , , • , , , , , • • , • , • ,

Calculated Frequency Ratio for I • ?/2 , , , ,

Quadrupole Coupling Constants for -f VaJues for Diatomic Halogen Molecules , • • , • , , ,

Ionic Character of Diatomic HaUdes Obtained From Nuclear Quadrupole Resonance Data , , • ,

Operator for EFG Tensor Components • • . . . . EFG Tensor Components for Cu • • , • • • • • •

Infrared Spectral of Bis(tetracarbonylcobalt)

Page

16

l?

20

23

26

JO 31

Derivatives • • • • • • • • • • • , • • • • • • 37-38

'!he Observed Absorbance Ratio and the Co-M-Co Angles in Bis(tetracarbonylcobalt) Derivatives of Sn and Ge Compounds , • , , • , , , • • • ,

NMR Spectra. . . . . . • • • •

Bond Angles (deg) and lengths (A) in Thiourea and Trls(thiourea)copper(I) chloride

Phyi=iical Properties of Bis(tetracarbonyl-cnbaJt) Tin Co~pound~. , . , , , •• , , ,

Elemental 'ral:ir.::is for Cnpper C~~pnunds, ,

. .

N~~ ~rP-~~ters fnr Bi~(te~t"e.~~~bonyJe~balt)­'t'ln(IV) Co111rounds. . • • • • • • • . • , • •

ObsP""."Ved Frequencies for Cu(I) Complexes

ObservP.d NQR ~e~uencies in Molybdenum

. . . Com'Pounds , . • , , • , , , , , . , • , . ' . . Experimental O~erved Frequencies Ratio and the AsYl1l!lletry Parameter Determined from

44

45

51

f.5

69

?4

Figure 22, • • • , • • , • , , • • • • • , • • ?8

v

Table XVIII

Table XIX

Table XX

Table XXI

Table XXII

Table XXIII

Table XXIV

Table XXV

Table XXVI

Table XXVII

Table XXVIII

Table XXIX

Table XXX

vi

NQR Data for Tin Compounds ••• • • • • • . . Orbital Populations in Fe(co)5 •••• , •••

Bond Direction of Cu-5 and Cu-Cl bonds w1 th Respect to x, y, z Axis System ••••••••

Angular Part of the Atomic Wave functions • •

Angular Contribution of One Electron in a Single Atomic Orbital ••••••••••••

The Total Contribution of One Electron in a

Page

80

83

93

94

96

Single Atomic Orbital • , •• , ••••• , , 97

The Contribution of a Single Electron in a Hybrid Orbital to the Z-EFG Tensor Component.

Estimated Orbital Population and Charge Density • • • • • • • , • • • • • • • • • • •

Ionic Contribution of Cl, Cu and S, to be qzz-EFG Tensor Component •••••••••••

Bond D1-rection of Cu-S, Cu-s Bonds with Respect to x, y, z Axis System ••••• . . . The Contribution of a Single Electron in a Hybrid Orbital to the Z-EFG Tensor Component,

Estimated Or.bital Population and Charge Densities , , •• , ••• • • f • • • • •

Ionic Contribution of Cl, S and Cu to q zz

• •

. .

99

101

105

107

108

110

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure ?

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

LIST OF FIGURES

Vectorial Representation of Nuclear Quadrupole Coupling, • • • • • • • • • • • , •

Frequency Ratio vs As)'llJlletry Parameter • • • • Electronegativity Difference vs Ionic Character

Page

6

19

as Detenained by Different Investigators • • • 25 Effect of Halogen Substituents on the Carbonyl Stretching Frequencies in XnRJ-nGeCo(co)4. • . 35

Schematic Representation of -Interaction Between Ge-Co and Co-CO Groups • • • • • • • • 36

Infrared Spectrum of ~lSn Co(Co)4 2-cs Ty'pe • • • • • • • • • • • • • • • • • • • • •

Infrared Spectnim of ~2sn Co(Co)4 2-c2v Ty'pe t • t t t t t t t t t I t I t t t I . . . A1 Mode. , , , • , • , , , • , , • , • • , , •

B Mode, 1

. . . . . . . . . . . . . . . . . ' . View Along the b-Axis Showing the Chain Type Structure in Tris(thiourea)copper(I)

40

41

43

43

chloride , . . . . . . , . . . . , , , . . . . 48

View of the Bis(thiourea)copper(I) chloride Chain Down the b-Axis Showing the Important Distances and Angles • • • • • • • • , • • , •

View Normal to Cu(l)-S(2)-CU(2) Plane of Orbitals Used to Make the 'Ibree Center De lo-calized Electron Pair Bridge Bond •••••••

Block Diagram of Superregenerative NQR Spectrometer • • • • • • • , , • • • • • • • •

50

53

61

Spectrum of a Superregenera t1 ve Spectrometer • 6J

NQ.R Resonance of 35c1 in c12sn Co(co)4 2 • • • 66

NQR Spectrum of 59co(.5/2-? /2) in c12sn Co(Co)4 2 67

'>9 I I NQR Spectrum of Co(l 2-3 2) in c12sn Co(Co)4 2 68

vii

Figure 18

Figure 19

Figure 20

Figure 21

Figure 22

Figure 23

Figure 24

Figure 2_5

Figure 26

Figure 71

viii

Page

NQR Spectrum of 63eu in Cu{etu)2c1. • • • • • , ?O

NQR Spectrum of 63eu in Cu(etu)4 2so4 • • • • 71

NQR Spectrum of ?9ar in Cu(etu) 2Br •••••• , ?2

NQR Spectrum of Mo Isotope in Mooc14 ••• , , • 15

Freq~ency Ratio vs Asymmetry Parameter Plot for 9co in c12sn Co(Co)4 2 • • • • , , • • • • 11

Resonance forms of Various legends, , • • • • • q -EFG vs Internuclear Distance ••• , • zz • • •

88

90

Orientation of Bonds in Cu(tu) 2c1 •. , , , , , 92

Structure of Tris(dimethylthiourea)Copper(!) Chloride. • • • • , • , • , , • • • • • • , • • 103

Orientation of Bonds in Cu(dmtu)3c1 • . , , • • lo6

INTRODUCTION

Since the first experiments regarding the properties of nuclei,

much interest has been centered on the interaction between the nucleus

and various environmental factors, particularly magnetic fields and

electric fields. As a result of such studies it has been found that

nuclei can possess magnetic dipole moments, electric quadrupole moments

and higher multipole moments. The fact that soma nuclei have an

electric quadrupole aoment that can interact with the surrounding

electric field is the basis for nuclear quadrupole resonance (NQJt)

spectroscopy.

A nucleus in any molecular environment is surrounded by electrons

and other nuclei. 'lbese electrons and nuclei are electrical in nature

and result in the production of an electric field at the nuclear site.

'Iha electric quadrupole moment of the nucleus may then interact with

the surrounding electric field in such a manner as to produce a discrete

set of energy levels. Transl tions between these levels may be observed

directly by application of radio-frequency energy of the correct

frequency. 'lhe frequencies of these observed transitions depend on

the quadrupole moments of the nuclei and the electric field gradient

(EFG) tensor components of the surrour¥11ng electric fields. Since

the nuclear quadrupole moment is a constant for a particular nucleus,

a knowledge of the EFG tensor components can be obtained exper!Mntally.

'lbese in turn can be correlated with the electronic distribution in

the molecule and hence with the type of bonding occuring in the mole-

cule. In this light, the quadrupole moment serves as a probe for

l

2

examining the internal electronic configuration of a molecule or of a

crystalline solid. Elucidation of the bonding properties of atoms in

solids may be afforded by judicious interpretation of the results of

such experiments.

The study of insertion reactions by inserting metal containing

groups into the Co2(co)8 to form. a metal-metal covalent bond was

initiated by Grahaa35, Infrared. and nuclear magnetic resonance studies

on these compoums have shown definite trends in the infra.red stret-

ching frequencies, the intensity of the infra.red spectra and the NMR

coupling constants. Since the observed NQR frequency of a particular

Co nucleus is sensitive to the local electronic environment a know-

ledge of the NQR frequencies and the electric field gradient tensor

components derivable from it can be correlated with the electronic

distribution in the molecule and hence with the type of bonding

occuring in the molecule. A correlation between the findings of NQR

studies and those of IR and NMR can further the understanding of the

nature of bonding in these compounds.

63cu and 65cu NQ.R resonance frequencies in cu2o and KCu(CN) 2

were reported in the early 1950•s, Since then, no other copper

resonances have been reported. The reported resonance frequencies for

both compounds were within the range of the available spectrometer so

this area constituted an open field for exploration. Due to the

inherent broadening of NQR transitions by the presence of a para-

magnetic species investigatioru> of Cu NQR frequencies were restricted

to Cu(I) compounds. A series of Copper(!) thiourea and substituted

thiourea complexes were chosen for investigation. 'nlese studies were

directed toward learning more &bout the nature of the bonding in these

compounds.

In an effort to extend the use of NQR to new systems several

molybdenwn compounds were investigated. Resonances which are

attributable to Mo nuclei were observed but precise assignment to a

particular isotopic species is impossible due to limitations on the

operating range of the spectrometer. 'nle limited nUJllber of observations

severely restrict the relationship of the observed frequencies to

bonding properties,

.REVIBV OF THEORETICAL CONCiiPTS

Al. NQR Energy I.eva1@ am Transitions.

The theory of nuclear quadrupole coupling in both atoms and

molecules has been extensively discussed in several general

monographs?,12,27,.54. In this section we will present a brief review

of those aspects of the theory which are of particular appllcabill ty

to the studies discussed in this dissertation. 'nle discussion will

thus be llmi ted to interactions in solids, nuclei of half integer

spin, and the case of no external magnetic field.

When a crystal containing an electrically asymmetric nucleus is

pl.aced in an oscillating magnetic field it may absorb magnetic energy

at certain frequencies determined by the electrical interaction of

the nucleus with its surrowdings. With an asynuaetric nucleus the

important electrical interaction is expressed as a product of the

gradient of the electric field at the nucleus am the quadrupole

moment of the nucleus. 'nlis interaction is known as nuclear quad-

rupole coupling. The Hamiltonian, H, describing the interaction

between a nucleus azxi the surrounding electronic charge may be

written as

r

where P (n ) is the charge density external to the nucleus in the e e

(1)

volume element dVe at position/\.e with respect to the center of the

nucleus and Pn(Vn) is the charge density of the volume element dVn

4

. '

5

within the nucleus at a positionlln with respect to the center of the

nucleus, 'nle vector~ is from dVn to dVe and 9en is the angle between

Ile and "n as shown in Figure l, By employing the law of cosines

l • (r 2 + r 2 + 2r r Cos 9 )-l/2 r e n en en

or l - (1 +(rn)2 - 2 51 Cos e )1/2 l r re re en re

An and expanding F.quation (3) in a power series in ;z:-, one obtains e

(2)

(J)

l • l [ 1 +?-Pl + ( rn) 2 P2 + .... J (4) r re e re

where P is the Legendre polynomial, i.e., e P1 111 Cos 9 en P • 1 (J Cos2e -l)

2 2 en

etc,

(5)

Substitution of F.quation (4) into equation (1) results in a series of

terms, the first corresponding to the interaction of the surrounding

field with the nuclear charge, the second corresponding to the inter-

action of the field with the nuclear electric dipole moment and the

third corresponding to an interaction of the field with the nuclear

electric quadrupole moment, 'Ille third term is the one of interested

in this study. In general a term in P,.t corresponds to a multi pole

moment of zl. , 'nle expression resulting froa substituting »1uations

(4) and (5) into equation (1) is,

6

7

P (r )/v 3J dV dV e e e n e v v e n

One must point out here that the condition for· the use of the

power series expansion to derive equation (4) is that An<\•

~uation (6) therefore is valid only if this same condition is f'ul-

filled. As a consequence of this condition, we have in effect

excluded all electronic charges which penetrate the nucleus. Such

an exclusion, however, does not pose a serious problem. It is known

that only s-electrons have a non-zero probability of penetrating the

nucleus, and because of their spherically symmetric distribution they

(6)

produce no observable interaction with the nuclear electric quadrupole

moment. One may reexpress ~uation (6) in terms of cartesian coordi-

nates by using the relationship

to yield

H • Q

~ xix i 1 e n

J P ( r )( JX ix • - Si .r 2) d v V n n n nJ J n n n

However, if one defines ~j and ( E)ij by the relationships

~ Pn(rn)(JXnixnj - ~ijrn2)dVn vn

-

(7)

(8)

(9)

8

and

(vE) • -1j

(10)

--Then,

1 Cl ..

HQ .., - 6 ~ Qij(9E)ij •Qi 'IE (11)

ij

where the double dot indicates a scalar product of two second rank

tensors, This may be verified by direct expansion of &luation (11)

using the first form of Equation (9) and the second form of F.quation

(10). ~uation (11) is the most generally used representation for H • - - Q It consists of two tensors Q (the quadrupole tensor) and VE (the

electric field gradient tensor) whose elements ~j and ( E)ij have

been defined by F.quations (9) and (10) respectively. By inspection ... -of F.quations (9) and (10) both Q and VE can be shown to be traceless

tensors.

The elements of the Q tensor can also be represented by the form42

(12)

where Ii and Ij are components of the nuclear spin operator I and C is

a constant scalar quantity. The arbitrary constant C may be expressed

in terms of a scalar nuclear quadrupole moment Q, which is a measure

of the departure of the nuclear charge distribution from spherical

symmetry. This scalar moment is defined by

l Q:e (13)

(YE)0 • ! V 2 zz

9

(vE)+ - - l6 (Vxz! i vyz)

(vE) "" * (V - V + 21 V ) .±2 GI u xx yy- xy

Any symmetric tensor may be transformed to a principal axis

(18)

system and thus be diagonalized. 1be diagonalized tensor components are,

where

(9E) • l V : l eq 0 2 zz 2

(9E)!l • 0

(tE).±2 • J6 (vxx- vyy)

q•lv e zz v -v 'l.· ~ v:y

zz

By using the convention

(19)

(20)

Iv l ~ tv I & Iv I c21) xx yy zz

'l may have values from 0 to 1. For 'l • 0,

V = V = ..l V ... l qe xx yy 2 zz 2 (22)

'Ihis corresponds to cylindrical (axial) symmetry about the z-a.xis of

the principal axis system. For the ease 't. • 1,

v ... 0 xx (23)

v - -v - eq zz yy

10

where the subscript m1 • I indicates the integral is carried out for

the nuclear state with the aagnetic quantum nWllber m1 •I. It can

also be defined by

where

(.IIIQzzlII) • C (IIl3(Iz)2 - I 2fII)

• C (312 - I(I + l)j

• C I (2J - 1)

therefore using Equation (12)

Q • eQ 1j I(2I-l)

(14)

(15)

(16)

At this point, it is of interest to point out that it is generally

considered that nuclei with spin I • O, 1/2 do not have quadrupole

moments. 'Ibis is incorrect however as Cook4 has pointed out. Nuclei

with I ... O, 1/2 can have quadrupole moments, but it is impossible to

observe a nuclear electric multipole moment of order greater than

2.f where l • 2I.

Let us now examine the electric field gradient tensor. '!be EFG

tensor elements given by F.quation (10) can be expressed in terms of

the v1j ... ~i ~X. where V is the electrostatic potential at the 1 J ..

nucleus. The units of v1j are cm-3. Since VE is a traceless

symmetric tensor and the IaplAce condition

v + v + v • 0 (1?) xx yy zz must be satisfied,7 there are 5 irreducible tensor components

11

nie asymmetry parameter, 'l• is therefore a measure of the departure

of the electric field from axial symJ11etry about the principal z-axis.

Next we will relate the observed frequencies to the quantities q, and

The quadrupole energy level matrix elements will be

(24)

where m takes the values ma I,I-1,•••••••·-I. Since

(ml~lm) .. m smm' (25)

(ml IX .! iIYlm') .. ((Im)(I+{m+1))1/ 2 S m,!l, m•J

where I , I , I designate the components of the spin angular momentum z y x

operator I, the only non-zero matrix elements become

(26)

and

The energies of the quadrupolar states are then g1 ven by

e2~ E ... 41( -1) ~ (1(1+1)-m{m+1)] 112 [(I+l)I - (m,!l)(m.:tZ~l/2 (28)

The quantity e~ is commonly known as the nuclear quadrupole coupling

constant,

For an axially symmetry case, i.e. 'l • O, the only non-zero

matrix elements come from F.quation (26) and the energies are given by

12

(29)

where

Inspection of nt,uation (29) shows that all levels, except that

one with m .. O, are doubly degenerate. 'Illus for half-integral spins

there are I+l/2 energy levels and for integral spins there are I+l

energy levels.

In order to observe transitions between quadrupole enerfzy' levels

one can in principle either apply an oscillating electric field, thereby

producing an electric field gradients at the nucleus which would interact

with the electric quadrupole moment of the nucleus or apply an oscill-

ating magnetic field to obtain an interaction of the nuclear magnetic

moment and the external osci llatJ_ng magnetic fielti. 'Ille forrner method

would require an electric field of 1014 volts/cm2 8 , 'lllls is too large

to be practical, 'Ille second method is generally employed, Since the

interaction involves an electromagnetic field a time-depend~nt

Hamiltonian must be considered, The time-denendent Hamiltonian re,re-7 senting this interaction is given by

H•(t) = - {'fi(H I +H __ T +HI) x x y-y z z (Jo)

where f is the magnetogyric ratio, h is Planck's constant, I , I , x y

Iz the components of the angular momentum operator I, and Hx' Hy, Hz,

the x, y, z - components of a linearly ~olarized oscillating electro-

magnetic field, 2H Cos wt, 'Ille transition probability which 1s

proportional to the matrix element <mlH' (t) I m•> 2 can therefore

lJ

be calculated with the aid of F.quation (25). For the. axially symmetric

case, 'l"'" o, the selection rules are •a "'" o, and Jj m • ±1. For

Am• o, transitions can be produced only by the z-component of the

magnetic field and involve no change in energy and are of no interest

here. For 4 m • ±1, the maximum transition probe.bill ty can occur only

if the Bohr condition

(Jl)

is satisfied.

1be frequencies of the quadrupole transitions for the axially

symmetric case are thus given by

E · -E ... m+l m Wm 'fl ., -?t (2 m + 1) (32)

Again there are I - 1/2 and I - 1 doubly degenerated levels for half

integer and integer nuclear spin respectively. If I and mare known,

then e~q may be calculated from the measured absorption frequency.

If m is not known, as is often the case for spin I""J/2, the values of

m and m may be uniquely determined by the ratio 1 2

w2 2 m2 + l (33)

For the case, I m J/2, there is only one transition frequency which,

when one assumes 1(. • O, is given by

w ... E±J/2-E±.1/2

'fl 6A - -1i - (34)

,. ; ,, ..

14

'!he problem of deriving expressions relating e~, 'l , and v,

becomes much more complex for the non-axially symmetric case. 'lbe

normal selection rules, 4 a • O, .:!: 1, no longer hold, In fact, for

values of 'l> 0.1, the probability for observation of transitions

corresponding to A 11 • .:!: 2 becomes quite large43, 'Ibis is because

of mixing of the pure magnetic dipoles states differing in a by .:!: 2.

Ve now not only have to consider the diagonal matrix elements Hmm •

(.m\HQtm) but also the off~iagonal elements Hm, m.!2 m~mlHQlm.:!:2) I

'!bus for I • 3/2, the energy matrix has the form

H3/2 3/2-E 0 H'.3/2,-1/2 0

0 Hl/2 1/2-E 0 Hl/2,-3/2 H -Q

H-1/2, 3/2 0 H -E -1/2-1/2 0

0 H -J/2, 1/2

0 H -J/2-3/2 -E

where H "' JA H = -JA and H = /3A 'l '!;J/2,"!;J/2 , +1/2.±1/2 +1/2!)/2 •

(35)

Upon reordering of the columns and rows, this matrix has the form,

HJ/2 J/2-E HJ/2,-1/2 0 0

H -1/2,J/2 H -1/2-1/2 -E 0 0 (36)

H = Q 0 0 Hl/2 1/2-E H+l/2-3/2

0 0 H -J/2-1/2

H -3/2-3/2

-E

15

'!he matrix is thus factored into two identical submatrices which

when solved given the energy levels,

E,!1/2 "" - JA (1 + i 2)1/2

E I = + JA (l+ ~)1/2

{37)

.:tJ 2 J

F.quation (J?) clearly demonstrates that the mixing of the magnetic

dipole states does not remove the m- degeneracy of the energy level.

Again, only a single frequency is observed for I • 3/2.

w {!3/2 -+ + 1/2) • e~Q (1 + ' 2)112 (38)

In a similar manner the secular equations for I • 5/2, 7/2 and

9/2 may be derived. They are tabulated in Table I.

The equations tabulated in Table I are not readily solvable with

the exception I = J/2. A numerical approach is generally used to

solve these secular equations5,26,2,43, Table II tabulates the

absorption frequencies in terms of "( , for 'l. ~ 0. 25, where a power

series expansion in 1t may be used.

From the ratio of the measured absorption frequencies it is

possible to obtain both 1l and e~ for nuclei with half integral

spins except for I = 3/2. For I = 3/2, Zeeman splitting of the

quadrupole frequencies must be studied in order to obtain 'l... and

e~ independently.

It was pointed out in the last paragraph that, for I • J/2,

both 1l and e2Qq can be independently determined from the ratio of

measured absorption fre11uencies. Cohen2 ha.s tabulated the eigenvalues

16

TABIE I

Secular equations for nuclei with half integral spin

I Secular F.quation Uni ts of Energy

3/2 l 2 - 3(3 +~) -o (, - E/A

5/2 £ 2 - 7(3+ rC>E-2(1- t 2) .,. o {. - E/2.A

7/2 E. 4 - 14(J+ 1',2) £ 2-6/.1.(1- 'l 2)l +35 (3+ 'l 2) = 0 l = E/'JA

9/2 l5 - ll(J+t,2) l J-44(1- 'l..2) l 2+ 4i' (J+1()2£ l. ::a E/6A

-t48(J+ 'l.2) (1- l 2) Q 0

17

TABLE II

Formulas for the Nuclear Qua4rupole Resonance Frequencies2

I• 3/2 w • ~ {l + 1t. 2/J)l/2 2fi

l.. 2 I• 5/2 wl - {~) {l +o.o 26'l2 - o.6:34t4) 20

- £.. 2

w {~) (1 + 0.2037 '{2 + 0,162 Jt4) 2 20

I• 7/2 wl • L ce~Qri) {1 + 50.865 , 2 - 10l.29't4) 14

w'.3 ... 1-{ 81') {1 - 2.8014 ~ 2 - 0.52781'4 ) 14

w2 ... L. { 8~) {1 - 15.867 ~ 2 + 52.052 'l4) 14

I• 9/2 w • L '*' {l + 9.0333 { 2 - 45.6911(_4) 1 24

w .. i (~) (1 - 1.J3811l,2 + ll.724'l..4) 2 24

WJ ... .1._ {~) (1 .. 0.1857'l.2 - 0,1233'l,4) 24

4 {~-) (1 - 0.08091(. 2 - 0,00437{.4) "4 = -24

J8

for the equations listed in Table I for values of 'l from 0 to 1 at

intervals of 0,1. Using these eigenvalues one can plot the calculated

frequency ratios vs 'l for the allowed transitions, From the inter-

cepts of these curves with horizontal lines constructed of the observed

frequency ratios, one obtains the value of 1l· Since a single value of

'l gives rise to a unique set of frequency ratios, the measure frequency

ratios should intercept the calculated curves in a vertical line,

Fig, 2 shows an enlarged section of such a plot, Table III contains

caleula ted frequency ratios for transl tions of interest in this work.

19

o. '.30 .- ..-. - - - - - - - - --- - - -

0.20

0.10

o.o O.J 0.5

Figure 2

Frequency Ratio vs Asymmetry Parameter

20

TABLE III

Calculated Frequency Ratio for I "'7/2

'l ~~=~~ ~ 2 ~ 2

0.1 2.89)88 1.50676 1.92059

0.2 2.63218 1.52453 1. 72655

o.J 2.)1877 1.54725 1.49863

o.4 2.02199 1.56812 1.28943

0.5 1.7662) 1.58098 1.11716

21

B. Interpretation of Nuclear Quadrupole Coupling Ila.ta

A variety of techniques have been considered for the interpre-

tation of NQR data7,l2,27, Two methods, the Townes-Da.iley approach

and a semiquantitative quantum mechanical calculation will be reviewed

as they will be refered to in later discussions,

'!be fundamental quantities determined by nuclear quadrupole

resonance measurements are e~ and 1l . The nuclear quadrupole coupling

constant, e~, ls the product of a nuclear property, Q, and a mole-

cular property, q. Since the nuclear quadrupole moment, Q, is a

constant for a given nucleus and in most cases is known, the molecular

EFG tensor component, q, can be determined from the measured nuclear

quadrupole coupling constant. On the other hand~ the quantity q could

be estimated if the charge distribution over the molecule were known,

Due to the non-availability of accurate wave functions q has been

rigorously calculated for only a few simple molecules11•13,

1.) Townes-Dailey Empirical Approach

Townes and Da.iley53 have proposed. that the value for eq in

a molecule can be expressed in terms of the analogous atolllic component,

(eq) t , 'Ibey defined (eq) t as the electric field gradient tensor a m a m component due to an electron in the lowest p-state outside the inner

closed shells of the free atom, 'Ille relationship between these two

quantities is

(eq)mol • f {eq)atm (J9)

where f is a factor that depends on the electronic structure of the

molecule and is refered to as the p-electron defect, Since Q is a

constant for a given nucleus and is independent on the electronic

22

charge distribution in the molecule, we can write

where (e~) 1, and (e~) t , are the experimentally determined mo - a 11

coupling constants for the molecule and the free atom respectively,

'lhe free atom nuclear coupling constant can be determined from hyper-

fine splittings in atomic beam epectra21.

(40)

If f can be written in terms of bonding parameters, information

rel.a.ting to the electronic distribution within the molecule will be

available from the experiaentally determined molecular coupling 7

constant. For a teI'llinally bonded atom f may be written

f - - (1 - 8 + d - i - "")

where s and d are the fractions of s- and d-hybridization associated

with fr -bonding, i is the fraction of ionic character and -rr is the

(41)

fraction of 11" -bond or multiple bond character. If one can reasonably

estimate or experimentally determine some of these parameters, NQR data

can be used to obtain the other, Except for transition elements

contributions due to d-hybridization can be ignored. Table IV lists

experimental values for the "p-electron defect" for several diatomic

halogens.

For a homonuclear diatomic molecule, one would expect the

hybridization, -rT -bond character and ionic character would be equal

in each atom, the f-value should therefore by unity or very close to

unity. 'lhis ls true for both Br2 and c12 with f-values of 1.00 and

0.99 respectively. For a hetronuclear diatomic molecule, hybridization,

2)

TABLE IV

Quadrupole Coupling Constanta29 and •f Values for Diatomic

Halogen Molecules

(e~)gas (e~)crys Compounds Nucleus MHz -f MHz -f

ClF 35Cl 146.o l.)J 144.0 1.29

C12 35Cl 108.50 0.99

ClBr 3501 103.6 0.95

79Br 876.8 1.14

Cl I 35c1 82.5 0.75 ?4.4 o.68

127! 29).0 1.28 )05.7 1.83

BrF 79Br 108.9 1.44

Br2 79Br 765.85 l.OO

I 12?! 2

215.6 0.94

"IT - bond character and ionic character should no longer be equal since

the electronegativity difference between the atoJlllS will cause an

unbalance in the electron distribution and the resulting f•valuea

should deviate from unity. Thia is observed for cu which has f. o.68 and f • l. :n for the Cl and I atoms respect! vely. 'lhis has been

explained.54 as resulting from a decrease of the p-electron defect on

the Cl atoms and an increase of the p-electron defect on the I atoms, - + 1,e. a partial Cl I structure. Townes and Dailey have also postulated

that for any terminally bonded halogen that is more electronegative than

the atom to which it ls bonded by 0.25 units, one should allow for 15~

a-hybridization. For electronegativity differences of less than 0.25

units no hybridization is allowed. Using this concept, along with

quadrupole coupling data, the ionic character of a bond that is reason-

ably well not expected to exhibit any "ft'" -bonding can be found from

the relation

(42)

Table V lists typical NQR data for some diatomic molecules and the

resulting ionic character calculated from this data.

'!he relationship between the ionic character as calculated from

equation (42) and the electronegativity difference between the elements

involved in the bond being considered is shown in Fig. J. Also included

in this figure are electronegativity curves due to Gordy12 and to

Paullng38. Gordy considered. a-hybridization to be zero in calculating

ionic character from NQJI data. Pauling used the empirical relation

1.0

0.8

1 o.6

0,4

0,2

25

Gordy

Pauling ·---Tovnes & Dailey

1 2

FIGURE )

Electronegativlty Difference vs Ionic Character as Determined by Different Investigators

26

TABIE V

Ionic Character of D1ato11ic Halides Obtained fro11 Nuclear Quadrupole Resonance Data&

Molecule e~ Ioniclty a-hybrid 1on1c1ty (MHa) assWAing no assumed calc.

s-hybrid (Gordv) (Townes & Dailey)

35c1 (atm) 109.74

BrCl 103.6 o.os o.o 0.056

!Cl 82.5 o.248 0.15 0.115

FCl 146.0 0.259 o.o 0.259

TlCl 15.8 0.856 0.15 o.831

KCl 0.04 1.00 0.15 1.000

RbCl 0.774 0.993 0.15 0.992

Cs Cl 3 0.973 0,15 0.968 79Br (atm) 769.76

BrCl 876.8 0.110 o.o 0,110

FBr 1089.0 0.329 o.o 0.329

11.Br 37 .2 0.952 0.15 0.944

Na Br 58 0,925 0.15 0,911

KBr 10.244 0.987 0.15 0.985 DBr 533 0,308 0,15 0.186

&Reference 29.

electro-negativity difference

0.20

0.50

0.75

l.?5

2.35

2.35

2.45

0.20

0.95

1.95

2.05

2.15

0,75

27

1 • 1 - exp (43)

where X and X are the electronegativity values for atoms A and B A B

forming the bond. It is clear that the differences between the two

NQR data approaches are small. A major difference exists between the

curves derived from NQR data and the curve due to Pauling. This is

because the ionic character as calculated from electronegativity values

alone neglects such effects as polarization and hybridization. On the

other hand, quadrupole coupling constants are related to the total

electronic structure of the molecule. 'Illus ionic character calculated

from NQR data are probably more accurate,

'lhe &llount of 'It -bonding involved in a compound increases the

p-electron defect, thereby affecting the quadrupole coupling constant

by decreasing its magnitude, 'l'o account for this, F.quation (42) can

be rewritten as

(e2QQ.) 1 = (1-i-s -1r) (e2Qq) tm mo a

'Ibe p-electron defect has now been related to the quad.nlpole

coupling constant, Since it is directly related to the population of

(44)

electrons in the p-orbitals we can next relate the quadrupole coupling

constant directly to the p-orbital populations, Nx, Ny and Nz• For

any atom irrespective of its mode of bonding it has been shown that20

2 e Qqxx

e2Qq yy

- - (N +N z 7 2

(45)

(46)

28

e2Qq • -{Ny ofttx - N ) (e~) Ziil z Z at (4?)

From these relations it is found that

(48)

For transition metal elements where d-electrons are involved in bonding

the populations of the d-orbi tals can be related to the quadrupole

coupling constant by

2 (49)

2). Semiquantitative Quantum Mechanical Evaluation of Electro-

static Field Gradient Tensor Components in a Molecule

In order to accurately calculate the electrostatic field gradient

tensor components in molecules, one must have the exact wavefunctions.

At present exact wavefunctions for complex molecules are not available.

It is possible however to make some reasonable and useful estimates of

EFG tensor components by using a molecular model based on convention~l

hybrid~ za.tion schemes, 'lhe method employed is to t"elit.te the c-ontrib,ltion

of electrons in a part.1.cular hybrid orbital to that of electrons in pure

atomic orbitals where the latter are represented by either hydrogen

like or Slater type orbitals. 'Ille contribution of a pa.rticular hybrid

orbl tal, ts then obtained by use of the conventional average value

expression

29

(50)

where gg• is a pair of cartesian coordinates and (qgg')op is the

appropriate EFG tensor component operator (Table VI). It is noted that

these components are products of angular and radial functions, the

latter of which are generally common for all atomic orbitals involved

in the calculation, S-orbitals will not contribute to the EFG tensor

because of their spherical symmetry, By using hydrogen-like wavefunctions,

the radial part is given by20

(....!) -rJ av

where n is the principal quantum number, l is the orbital quantum

(51)

number, z• is the effective atomic number and a 0 is the Bohr radius, By 20 using the rules given by Kauzma.nn for calculation of screening constants

the effective atomic number of any atom can be estimated. The EFG tensor

components for a single electron in the bonding atomic orbitals of a Cu

atom as calculated by using equations (50), (51) are tabulated in Table

VII. Since the Cu atom will be discussed at considerable length later

it will be used as an example to illustrate the method of calculation.

As a start one employs any available bonding information to construct a

suitable model, Consider the <T -bonding orbi ta.ls of the Cu atom to be

4 spJ-hybrid orbitals of the general forms

JO

TABLJr VI

Operators for EFG Tensor Components

q • {~)(J sin29coa2f6 -1) xx rJ

Clxy • <;,> {J s1n2ecosls1n cJ )

~z • C!J') {J s1n9cosecostf )

q • {~) (J s1n2es1n2~ -1) yy rJ

q • (8 ~> (J s1n8cos9sinf' ) yz r.1

31

TABLE VII

EFG Tensor Components for Cu

Contribution of a single electron in a single atomic orbital

Atomic Orbitals EFG tensor component X lo-14 esu ca·· -3

Clµ qyy qzz

4 p -6.84 x 3.42 J.42

4 p y J.42 -6.84 J.42

4p J.42 3.42 -6,84 z

J)d 1.16 1.16 -2,)2 z

3 d 2_ 2 x y -1.16 -1.16 2.J2

3 dxy -1.16 -1.16 2.32

3 dxz -1.16 2.J2 -1.16

3 d yz 2.32 -1.16 -1.16

32

~1 -a 1s + blPx + c1Py + dlPz

~2 .. a 2s + b2Px + c2P1 + d2Pz

(52) '¥3 -a s + b p + c p + d p

J J x 3 y 3 z

'P4 - a4s + b4Px + c4Py + d4Pz

'lbese hybrid orbitals are oriented relative to an arbitrary cartesian

coordinate system which may be chosen to coincide with the crystal axis

system or may be related in some symmetrical way to the bond directions,

'!be nucleus (Cu atom) in question is located at the origin. If the

directions of the hybrid orbitals do not coincide with bond directions

one can rewrite the hybrid orbitals in the form,

• 2 a a s + b Cos J. P + C Cos /3 P + d Cos { P T 2 2 2x 2 2y 2 2z

where el i' /Ji, { i (i = 1,2,J,4) are the angles rel.a.ting the

directions of the bonds to the cartesian, axis system. Since the

hybrid orbi ta.ls will be normalized there are eight relations

2 2 2 2 2 2 ..12 a1 +bi Cosal i + c1 Cosf'1 + d1 Cos l'i • 1

I a 2 • 1 \ 1

(53)

JJ

t.. b2 cos.C 2 (.54) - 1

~ i

2 2 ~- c Coe pi - 1 '

2 { 2 Ii d Cos 1 ... 1

'Ihese eight equations can be solved simultaneously to yield value for

the constants. The final forn of the hybrid orbitals then becomes

'f' .. a I s +bl' PX + c ' p + d I p 1 1 1 y 1 z

, a a I s+b• p + c ' p + d I p

"' 2 2 2 x 2 y 2 z

(55) ~'3 -a ' S + b I p + c ' p + d I p

3 3 x 3 y J z

·'f4 ... a ' S + b I p + C I p + d I p 4 4 x 4 y 4 z

'Ihese functions are used to ca lcu late the EFG tensor components by

using equation (50) along with the appropriate EFG operators. The EFG

tensor components due to a single electron in each hybrid orbital is

thus calculated. Since, in most cases, an atom will contribute either

more or less than one electron to a bond with other atoms, we are

confronted with the problem of assigning the number of electrons present

in a given hybrid orbital. TI11s is done with the aid of auxillary

infonnation such as the ionicity of the bond as calculated from electro-

negativity differences and the possibility of the existance "11-character,

Finally, the sum of the contributions of each hybrid orbital to the EFG

tensor components is determined and the results compared to experimental

values,

LI'I'ERA'ruRE REVIEW

A, Bis(tetraearbonrlcobalt)tin Derivatives

The insertion of tin(II) halides into cobalt carbonyl to form a

metal-metal covalent bond was first reported by Heiber23 in 1957,

later, in a series of papers, Gre.ham16•18•19•30- 37 •45 •52 extended this

investigation to include all elements in the fourth group with a number

of transition metal carbonyls such as Mn, W, and Rh, Most of the

compounds reported were prepared by conventional halide displacement

or by direct reaction of MX4 with metal carbonyls, . They are crystalline

materials, stable under a nitrogen atmosphere but decompose on exposure

to air,

Infrared studies on a number of bis(tetracarbonylcoba.lt) derivatives

of tin and germanium revealed that the C!!O stretching frequency tends to

shift to higher values as the electronegativity of the halogen substi-

tuents on the metal atom increases (Table VIII), Furthermore, if one

plots the frequencies of the A1 or A1• CO stretching modes against the

Pauling38 electronegativity of the halogen substituent a linear relation-

ship is obtained, Figure 4 illustrates this for the germanium compounds,

These trends are true not only for bis{tetracarbonylcobalt) compounds

of germanium or tin, but also for mono and tris(tetracarbonylcobalt)

derivatives of all elements in fourth group except carbon. These

trends have been explained in terms of substituent effects on

1T -bonding in the molecules, Figure 5 represents schematically the

1T -interactions between the metal a toms and the ligands. There are

two electrophillc sites, Ge and CO, competing for the electron density

2120

2070

2020

3.5

.t X halogen

FIGURE 4

A' 1

Effect of Halogen Substitutents on the Carbonyl Stretching Frequencies in X R3 GeCo(co)4 n -n

'

36

Figure 5

Schematic Representation of 11'-Jnteraetion Between Ge-Co and Co-CO Groups

TABLE mt Infrared Spectra of Bis(tetracarbonylcobolt) derivatives&

Compounds with c2v Symmetry

Al Bl 2Al + 2Bl + B2

c12ee(co(co)J 2 2117 2100 2058 20.54 2044 2026 2016

I2Ge (Ch (C0)4) 2 2113 2096 20.54 20.51 2042 2025 2013

(CH3) 2Ge [co(C0)4l2 2098 2081 2033 20-z? 2019 2006 1997

c12sn(Co (co)J 2 2114 2097 2056 2052 2040 2023 2016

Br2sn (co(co)4] 2 2113 2096 2055 2050 2040 2036 2016 '.:d If3n f co(co)4] 2 2110 2093 20.53 2048 20Y1 2021 2012

(CH3) f3n (co(CO) 4] 2 2095 2078 2031 2024 2013 2002 1992

4> 2sn [Co(co~J2 2095 2080 2033 2039 201B 2009 1995

(CH2-cH) 2Sn [co(C0)4] 2 2097 2080 20Y1 2028 2018 2009 1998

TABLE VIII (con•t)

Compounds with C A ' A" JA• + JA" Symmetry 8 1

I ( CH3)Ge [co(CO) 412 2106 2089 2046 2040 20)0 2024 2014 2000

Cl(CH3)sn[Co(C0)4]2 2104 2088 2044 2038 2022 2017 200.5 1996

c115n (co(co)4J 2 2105 2088 2045 2039 2030 2021 2014 1999

Cl(n-C4H9)Sn[Co(Co)J 2 2103 2086 2044 2037 2024 2018 2007 1996 ~

Cl(CH2mCH)Sn [co(C0)4J2 2101 2088 2046 2040 2029 2022 2017 2000

8Reference J6

39

in the filled cobalt 3d-11'-orbi ta.ls. Any increase in 11" -bonding of

either the Ge or CO must occur at the expense of the other. Suppose

the 1T -bonding between germanium and cobalt increases, the 1'"-

bonding between carbon aonoxide and the cob&lt atom must accordingly

decrease. Consequently, the carbon monoxide will experience an

increase in bond order and hence in the magnitude of the C~ stretching

frequencies. 'Ille inductive effect of the subst1tuents on the germanlwn

atom will increase or decrease the electron affinity of the empty 4d-

germanium orbitals. For a highly electronegative substituent such as

chlorine, the electron affinity of the empty 4d-11" orbitals increases.

'lhis in turn provides the germanium atom with a greater ability to

accept electrons from the cobalt atom to form a stronger 11" -bond,

'lllis produces the effects cited above. Graham also observed from the

infrared studies that there is a considel"l\ble coupling of the vibrational

modes of the two Co(C0)4 groups across either a germanium or a tin atom

in the bis-compounds. Such a coupling can be simply explained by

symmetry considerations. For molecules of C or C symmetry, group 2v s

theory predicts seven infrared act1 ve bands (JA1 + 3B1 + B 2) for the

C case and eight infrared active bands (4A1 + 4A") for the C case, ?v s

However, on the basis of the "local symmetry" of the two equivalent

Co(co)4 groups, one would expect only three or four frequencies that

are infrared active if one assumes there is no coupling between two

equivalent Co(CO) groups. 4

'!he fact that seven or eight absorption bands were observed

(Figure 6, 7) is a clear indication of coupling between the vibrational

modes of the two Co(co)4 groups across the germanium or tin atom.

40

100

s:: 0

oM Ill (/) .... E u. s::

!: 50 ll!

0

FIGURE 6

Infrared Spectrum of r/clSn Co(co)4 2 - C6 Type

41

too

8" i:: 0

<M U) U) 60 ort

"' II) ~

~ 40 ~'

?O

2120

FIGURE 7

Infrared Spectrum of ¢ Sn Co(co)4 - C Type 2 2 2V

42

From infrared intensity measurements it was found that the

intensity of the A1 mode {Figure 8), in which the CllQ dipoles of the

two Co(CO) tend to oppose each other, was, in general, weak and 4

tended to increase as the Co-M-Co angle decreased. '!he intensity of

the B1 mode {Figure 9) where the dipoles tend to reinforce one another

increases wt th increasing Co-M-Co angle, If one assumes collinear! ty

of the dipoles17 on the cobalt atoa and the M-Co bond then a simple

geometrical relation can be formulated,

A ;!J:. • Cot (~)2 ""Bl 2

where A ls the absorbance of the indicated band and 9 is the Co-M-Co

angle, '!he observed absorbance ratios and the Co-M-Co angles for

bis(tetracarbonylcobalt) derivatives of germanium and tin are given

in Table IX, It is interesting to observe, the "bond angle" tends to

increase as the e lectronega ti vi ty of the subs ti tu en ts increase, One

(51)

must not take the "bond angle" 11 terally, The trends, however, are as

would be expected on the basis of Bent•s rule1, i,e,, tha.t electro-

negative substituents tend to free the a-character of the tin or

germanium atom 0- -bonding orbitals for use in the metal-metal bonds,

with a resulting increase in the metal-metal bond angle. NMR studies

of chemical shifts9,l7,lO and proton-tin coupling constants in methyl

tin derivatives suggest that this is indeed the case. Table_X lists

the NMR results, 10 Kaesz , et.al. suggested that the coupling of the spins of the

adjacent nuclei by means of bonding electrons is expected to be propor-

43

FIGURE 8

A mode 1

R 2

I

FIGURE 9

B mode 1

,- ·..r.i:.. • .

44

TABLE IX

1he observed absorbance ratios and the Co-M-Co angles in Bis(tet:racarbonyl cobalt)

deri va ti vea of Sn and Ce compounds

Compounda AAlfBl Cot 9 /2

Cl2Ge(Co(C0)4h l.04xl0-2 0.102

I{39 [co(co)4 ]2 014.1 o.64 (ctt3)2Ge [co(co)4] 2 0.38 0.618

Clln [co(co)4] 2 l,Olxl0-2 0.1005

Br2sn [co(co)4l 2 l.OJxl0-2 0.1015

I Sn [co(co)4] 2 2 0.365 o.604

(CH3)2Sn [co(C0)4J 2 0.370 o.608 (~2sn [co(co)4] 2 0.515 0.718

(CH2-CH) 2Sn [co(C0)4J 2 o.460 o.678

I(CH3)Ge [co(CO\l2 0.290 0 • .538

Cl(CH3)Sn [co(CO\] 2 0.300 0.548

Cl~n(Co(Co)4) 2 0.330 0.574

Cl(Cl-C4H9)Sn (co(C0)4] 2 O.JJO 0.574

Cl( CH =CH)Sn [co( CO\] 2 2 0.4JO 0. 6.56

e

169°40•

115, 15•

117 • 28'

169°28 1

169°38•

116°16•

116°36•

105°22•

110°16•

122°34•

123°28•

120°42·

120°42•

112°36•

45

TABLE X

NMR Spectra&

Compounds J(ll7sn..CH ) 3

J(119Sn..CH3)

(CHJ)Jsn Co(C0)4 9.31 50.6 52.6

(CH3) 2Sn [co(C0\12 8.88 4J.6 45.7

CH Cl sn[co(CO) ] 3 4 2

8.48 42.2

b cH3sn (co( co) J 3 8.98 JJ.O

&in cnc13 solution b Reference 33

. '

46

tional to the densities of the bonding electrons at the respective

nuclei. Thus, the coupling of the nuclei will be greater when there

is a larger amount of s-chara.cter in the overlapping hybrid atomic

orb! tals that form the bond between them. For the proton-tin system,

the interaction or coupling takes place between two bonds, C-H and

Sn-C. Since the variation is small between J ( c1JHJ) • 129. 2 cps for 35 lJ 36 (CH3)2SnMn(Co)5 and J(C H3)• 128.0 cps for (cH3)4sn, it is reason-

able to assume that the hybriJization of the carbon atom remains

constant. The observed change of the proton-tin coupling constants can

then be attributed to the variation of the s-chara.cter in the tin s-p

hybrid orbitals which are directed toward the methyl groups. 'Ihe

observed. decrease in the coupli:lg constants as the number of methyl

substituents decreases suggests that the tin er-orbitals bonded to

the cobalt atom is enriched in s-chara.cter, in accordance with Bent's

rule.

B. Cu(I) thiourea and substituted thiourea complexes

1. 'Ihiourea complexes

Cu(I) thiourea complexes were prepared in the early 1900's by

a number of German workers 40 •41• It was proposed by Rathke 40 •41 that

the bonding in these complexes was through the sulfur atoms with a

structure of the form

NH 2, NH

2 /

C S Cu-Cl

47

X-ray crystal structure studies have since revealed a21 number of

different types of metal-sulfur bonds in compounds of this type.

Typical of this are the observations of the occurance of an electron

deficient bond in Cu(tu)9 ( N03)4 and of sulfur brid$1ng in Ag(tu) 2c1.

Yamaguch159 and Irvtn€'0 reached similar conclusion that the

complexes are coordinated through the sulfur atom Via infrared studies

on a number of complexes of thiourea with various metals, Their

conclusions were based on the weakening or complete disappearance of

the band at 1083 cm-land the lowering of the frequency of the band

at 730 cm-1• Yamaguchi suggested that the weakening of the band at

1083 cm-1 was due to the decreasing of C=S stretching frequency and

an increasing in N-C-N stretching frequency. The lowering of the

frequencies was contributed. to the decrease of double bond character

of the C""5 bonds. Their conclusicn was substantiated by subsequent

X-ray studies on Zn(tu)2c12,55 Cu(tu) 3c1,56 and Cu(tu) 2c148•49.

The crystal structures of both the tris and bis(thiourea)

Copper(!) chlorides are of interest for subsequent nuclear quadrupole

resonance interpretation, and therefore warrant a detail description. 21 Knobler et.al. found that tris(thiourea) Copper{I) chloride crystallizoa

in the tetragonal system with a = 13.41 A and c = 13. 76 A. The structure + consists of long spiral chains of Cu(tu) 3 1.ons (Figure 10) inter-

spersed with chloride ions. It was found that the copper atom is

surrounded by four sulfur atoDIS, two of which are shared by two copper

atoms and serve as links to form the chain structure. The internuclear

distances of the Cu-S bonds are 2.13 A, 2.34 A, 2.JJ A and 2.42 A, 'lbe

former two are the Cu-S distance:; of the unshared sulfur atoms and the

48

FIGURE 10

View Along the b-Axls Showing the Chain Type Structure in Tr!sthiourea Copper(!) Chloride

49

latter two are for the shared sulfur atoms distances. 'Ihe configuration

of the copper atom is a slightly distorted tetrahedron with S-Cu-S

angles varying from 101 degrees to 108 degrees,

Table XI lists the structural parameters of the thiourea ligands

in the crystalline complex along with those of uncoordinated. thiourea.

'lbree ligands have, at least oneC-N bond length that is shorter tha.n

the C-N bond length in the uncoordinated thiourea and is closer to the

CaN double bond distance of 1.21-1.28 A, while ligand I has two almost

equal C-N distances. 'nle short c-N bond lengths were rationalized in

terms of resonance structures of the llgand25,

+ + NH2~H2 .._, NH2 • ~2 - NH24 • NH2

s s- s-

It is not certain what the reason is for the exlstance of

unequal C-N bonds in both ligand II and III which are single coordinated,

One suggested explanation was interaction of the anions which causes

distortion of the chain, It was further pointed out that all the ligands

lie in the same plane within experimental error,

'Ihe crystal structure of the bis(thiourea) Copper(!) chloride 48,49 was reported by Amma et, al, Both the geometry and the bonding

were somewhat novel for Cu(I) compounds, The Cu atom is located in a

near trigonal planar environment surrounded by sulfur atoms from three

different thioureas, 'nle sulfur atoms at two of each of the three

trigonal positions are shared by different copper atoms to form infinite

spiral chains (Figure 11) similar to the trt!l-compound, Ea.ch Cu atom

has associated with it a long axial Cu-Cl distance, The bonding

'iO

Figure 11

View of the Bis(thiourea) Copper (I) chloride Chain Down the b-a.xis Showin~ the Important

Distances and Angles

.··· :···

Thiourea

I

II

III

51

TABLE XI

Bond angles (deg) and lengths (A) in thiourea and tris(thiourea)Copper(I) chloride

1.705

1,797

1.820

1.832

C•:tf

1.313

1.241

1,287

1.458

C-N•

1,313

1.290

1.398

1,195

S-C-N

122.5

114.8

127.4

109,8

S-C-N'

122.5

114.2

111.4

123.8

115.0

129.9

121.1

124.3

52

between Cu-Cl is considered as ionic. It was pointed out that the

Cu-Cu separation alternated between a long and a short distance with

an accoapaning "broad" and "sharp".Cu-S-Cu bridging angle, '!he short

metal~etal distances with the sharp bridging angle was explained by

the formation of a three-center electron pair bridging bond,.resulting

from the overlap of sp2 orbitals from each Cu atom and a p-orbital fro•

the s atom (Figure 12). 'nle formation of the "broad" bridging angle

and accompanying larger bond lengths was suggested as being due to the

overlap of 1' -bonding orbitals on both the Cu and S atoms.

2. Substituted thiourea Copper(!) CO!!lplexes

Morgan and Bursta1129 prepared a variety of Copper(!) ethylene

thi ourea salts • 'lhese include 1

Cu(etu)4No3 --- Colorless prismatic crystaµ>

eu2(etu)5(No3)24H20 --- White, six-sided prismatic

Cu(etu)3 2so4 --- Colorless, three-sided prismatic

Cu(etu)3Ac --- Colorless, eloagated plate

Cu(etu) Cl --- Colorless, rhombic 2

Cu(etu) 0 --- Floculent, white 2

etu • ethylene thiourea

'nle structure of these compounds was considered to involve Cu-S bonding.

A limit of four-fold coordination was recognized. In the limit of four

coordinated ligands per Cu the Cu will have acquired eight electrons,

giving it a krypton structure. Neither crystal structure analysis

nor infrared spectroscopic studies been done on any of the ethylene 25 thiourea complexes, except Cu(etu)4No3 •

N----- - -

I I

I

I

I I

I I

53

\ \

\

\ \

\

_,... -------- -- ----- - - --.~~--Curi) 2. 98 A Cut&)

FIGURE 12

View normal to Cu(l)-s(2)-cu(2) plane of orbitals used to make the three center delocalized electron pair bridge bond.

EXPERIMENTAL

A, Preparatory work

1. B1s(tetracarbonxlcoba.lt)t1n Compounds

All of the compounds studied were prepared by the procedure

g1 ven by Patmore and Graham. All reactions were carried out under a

static nitrogen atmosphere and the solid products were sealed in screw

cap vial.8 using teflon tape to 1111.nimize exposure to air, '!able XII

lists the physical properties of the compounds prepared along with

the results given by Graham. In ad.di ti on to the use of color and

melting points to ascertain the composition of the products the infrared

spectra of all three compounds were found to be in complete agreement

with those published by Graham.

2, Copper(!) thiourea and substituted thiourea complexes

a. Tris(N.N. 1 -dimethylthiourea)copper(I)chloride1. 'lhis compound

was prepared by dissolving 5.25g N,N 1 -d1methylthiourea (Aldrich Chem-

ical Company, Inc,, Cat, ID18,870-0) in a minimum amount of methanol

and reacting this solution with a solution of 8.55gm CuC12·2H20 prepared

in a similar manner, '!he mole ratio of the two components was 411.

'Ille solution was concentrated by boiling until a yellow precipitate

of sulfur formed, 'Ibis was removed by suction filtration, Further

concentration yielded a yellow viscous liquor which on cooling and

stirring very slowly formed creamy white crystals. 'lhe crystals were

filtered by suction, washed with acetone and air dried, 'lhe resulting

solid was recrystallized with some difficulty and loss from methanol,

Table XIV list the C,H,N, elemental analysis for this and other copper

compounds,

55

TABLE xn Physical properties of

bis(tetracarbonylcoba.lt)tin compounds

Color

'lhis work Graham

Co(C0)4)2 SnC12 orange-red orange-red

Cl</Sn Co(Co)4 2 yellow yellow

f2Sn Co(Co)4 2 yellow-orange ye How-orange

Melting Point

This work Graham

104 105

128-1:32 128-131

71-?3 71-73

56

TABLE )([II

Elemental analysis for copper compounds

Cale. Found

c H N c H N

eu2(etu)6so4 25.62 4.29 19.99 25.53 4.'.33 19.96

Cu(etu)2Br 20,80 3.50 16.4 21,18 ).64 17 .35

Cu(etu)2c1 25.15 4,22 20.02 24.25 4,03 19.87

Cu(tu)2Br 8.10 2,69 18.70 8.15 2.93 18.00

Cu(tu)lo3 8.62 2.87 24.12 8 • .54 2,96 23.97

Cu(tu) 2Cl 9.45 3.52 22.02 9.87 3.63 21.87

Cu(etu)2c104 20,6; 4.85 16,10 20.51 4.55 15,88

Cu(mtu) Cl 20.94 5,21 24.41 20,66 5,11 24.35 4 Cu(dmtu) Cl

3 26.Z? ;.as 20,43 26,36 6.oo 20.26

etu • ethylenethiourea

tu - thiourea.

dmtu m dimethylthiourea

mtu • aethylthiourea

57

b, Tetra.kis(N-methylthiourea)copper(I)chloride1 This compound

was prepared by the method described by Urbanik, A solution of O.J6g

N-methylthiourea (A.c.c. M8460-7) (o.04 mole) in a minlmwn amount of

methanol was to a solution of 1.71g CuC12•2H2o (0,01 mole) in a minimum

amount of methanol, 'Ihe procedure for concentration described in

(a) above was followed, Concentration yielded pale-creamy white crystals.

These were filtered by suction, washed with methanol and air dried, This

complex was insoluble in most common organic solvents, including ethanol,

acetone, benzene, carbon tetrachloride, and chloroform, It was also

insoluble in water, but was very slightly soluble in methanol, The

melting point range of this compound was 145-147.

c, Bis(ethxlenethiourea)copper(I)chlor1de1 This compound was

prepared by a method suggested by Morgan and Burst.all, o.04 moles of

ethylene thiourea (EKC-No, 5950Jwas in dissolved in 100 ml of water,

0,01 moles of cupric chloride was added to the aqueous ethylenethiourea

solution, '!he solution was concentrated by boiling-until crystals

began to fomr. 'Iha solution was cooled to room temperature filter.

'!he compound was recrystallized from water.

d. Bis(ethylenethiourea)copper(I)bromide1 'll"lis compound was

prepared by the lllllnner described in (c) above with the substitution of

CuBr2 for CuC12•

e. Tetrakis(ethylenethiourea)copper(I)nitrate1 This compound

was prepared by the manner described in (c) above with the substitution

of Cu(No3)2 for CuCI2•

f 1 Tris(ethylenethiourea)copper(I)sulfatea '!his compound was

prepared by the method described in (c) above with the substitution of

Cuso4 for CuC12•

g, Tris(thiourea)copper(I)chloride1 A minimum amount of boiling

water was used to dissolve 0.45 mole of thiourea, (EKC No, ~9~5 ).

'Ibis solution was added to a solution of 0,1 mole Cuc12°2H20 prepared

in an identical manner, '!be resulting reaction mixture was filtered

hot to remove sulfur and cooled in an ice bath. '!be white crystals

which foraed were filtered by suction, and recrystallized from hot

water,

h, Bis(thiourea)chloride. bromide and nitrates All of these

compounds were prepared by the procedure described in (g) with the

amount of ligand being limited to 0.35 mole,

59

B. Instrumentation

1, Superregenerat1 ve Zeeman-Modulated Spectrometer

In order to provide a suitable means for searching for nuclear

quadrupole resonance lines, a source of radiofrequency power which is

both reasonably stable and sensitive over a wide frequency range is

needed. For searching purposes a superregenerative oscill.Ator-

detector system is usually employed,

'nle underlying principle of the superregenerative spectrometer 28 14 has been described by a number of authors ' • 'nle system used has

been described by Croston6. 'Ihe general operation will be briefly

reviewed, 'ftle unique characteristic of a superregenerati ve oscillator

ls the periodic quenching of the oscillations. This results in there

being a period of time during which the oscillations are cut off

followed by a period when the oscillations build up from a low level

to some maximum value, Such repetitive build-up of oscillations is

accomplished by means of either large negative pulses being applied

to the grid of an ordinary cw oscillator tube (external quenching) or

the RC time constant in the grid circuit is made sufficiently large

to allow the necessary negative voltage to develop on the grid before

the capacitor is discharged (internal or self-quenching). When these

periodically quenched oscillations are subjected to absorption by the

sample, the maximum oscillation amplitude is decreased. At the same

time, if the relaxation time for the nuclear signal in the sample is

shorter than the quench period, oscillations build up from the noise

rather than the decaying nuclear signal and incoherent operation

results. On the other hand, if the relaxation time for the nuclear

60

signal is longer than the quenching period, the oscillations then build

up from the tail of the dying nuclear signal and coherent operation

results, '!he maximum amplitude of the oscillation is then effectively

determined by the nuclear absorption.

In order to be able to detect the difference in oscillation

amplitude between absorbing and nonabsorbing conditions, Zeeman modu-

lation is provided. A. periodic magnetic field created by a square wave

of current is applied to a Helmholz magnet surrounding the sample coil

of the oscillator system. The absorption of a powdered sample will be

broaden and its intensity reduced to zero by the magnetic field if the

magnetic field is on and the nuclear absorption will be present if the

magnetic field ls off. 'llle effect of the modulation is to amplitude

modulate the rf-signal. '!he oscillator output is then filtered to

remove the rf- and quench frequencies and the remaining component,

which is at the modulation frequency is amplified and recorded,

A block diagram of the spectrometer circuit is shown in Figure

13.

2. Method of frequency measurements

Two methods have been used for frequency measurements and have

given consistent results on both the samples studied and known com-

pounds, (1) Frequency measurements were ma.de by setting the oscillator

on the center of a resonance line and converting the oscillator to cw

operation by the imposition of a large de voltage to the control grid.

The oscillator frequency was then measured with a Hewlett Packard

52451 frequency counter, For known resonance this method gives

Pre-

Amplifier

Filter

Monitor

61

Oscillator

Detector

l

Phase-sensi-ti ve

Detector

1 Recorder

FIGURE 13

Frequency

Counter

Ref. __ .,. Oscillato

Modulator

315 Hz Oscil-lator

Block Diagram of Superregenerative NQR Spectrometer

62

agreement to .:!.Q.002MHz, (2) The resonance frequencies were also

measured by using a system involving an external reference oscillator

and a spectrum analyzer. The multiple sideband spectrum of the super-

regenerative oscillator which results from the quenching action, and

is illustrated in Figure 14, is observed on the screen of a high

resolution spectrum analyzer which ls coupled to the oscillator, The

center frequency, f , is the frequency to which the oscillator ls c tuned and is the frequency of interest in any measurement, The side-

bands are separated from f by multiples of the quench frequency, f , c q

and exhibit decreasing amplitude as the order of the sideband increases,

A vartable frequency oscillator (VFO) is also coupled to the high

resolution spectrum analyzer and serves as a reference frequency source.

When the reference frequency fr is coincidental with the frequency fc,

as observed on the analyzer oscilloscope, the former frequency is

measured with the frequency counter. The limitation on the frequency

measurements is that of setting the spectrometer on the peak of an

absortion line, This is of the order of l~ of the quench frequency

or .:!.Q,002MHz.

63

Amplitude

l'Jl

i ()

i 0 H)

Sit tll r:: i a ~ 'rd § ~ l"l al

Pl ~ c+ ..... ~ CJ)

"d C1I () c+ '1 0 ;;I al c+ Ill l"f

C, NQR Data

1. nie bis{tetracarbonylcobalt)tin compound were searched for the

nuclear quadrupole resonance frequencies over a range of 5-40 MHz at

room temperature. Table XV gives the compounds investigated and the

resonances found, Figures 15 through 17 show typical resonance patterns

obtained. nie cuprous oxide resonance at 26,020MHz was used to check

the sensitivity and the resolution of the spectrometer. 'Ihe lowest

pair of resonances for the chlorophenyl and diphenyl compounds could

not be found due to their low intensity, All of the compounds studied

had very low intensities. In order to improve the observed intensities

the sample coils were wraped tightly around the samples to maximize

the coil filling factor. Also very low scanning speed and long time

constants were used to obtain maximum response,

2, nie copper(!) coordination compounds were searched for nuclear

quadrupole resonance frequencies over a range of l0-60MHz at room

temperature, nie compounds studied along with the observed frequencies

are g1 ven in Table XVI. 'Ihe intensities of the observed frequencies

varied, Both Cu(etu)2c1 and Cu(etu)4 2(so4) were also searched at

liquid nitrogen temperature by using a cold finger dewar.

18 through 20 show typical resonance patterns obtained,

Figures

TABIE XIV

NQR Parameters for Bis(tetra.carbonylcobalt)t1n (IV) Compounds

590 o a Resonance Frequencies (MHz)

Compound 1 (s/N) 2(s/N) J(S/N)

Cl2Sn [co( CO\] 2 10.85:3 (5) 21.24:3 (8) 31.926 (8)

10.516 (5) 20.607 (8) J0.988 (8)

c1¢;n [co(co)J 2 b lB.356 (2) 27.500 (2)

b 18,100 (2) 27.250 (2)

¢2sn{Co(co)4J2 b 16.075 (2) 24,219 (2)

b 16.0JO (2) 24.089 (2)

aExperlmenta.l error for all frequencies is !_(),004 MHz, b Not observed,

:35c1 Resonance

Fr{uency (MHz) S/N)

17. 676 (3)

17 .150 ('.3)

b °' \J\

66

FIGURE 15

NQR Spectrum of 35c1 in c12sn Co(co)4 2, 2_5°C,

0.2MHz/hr Scan Speed, 3 sec. Time Constant

67

FIGURE 16

NQR Spectrum of 59co(5/2-7/2) in c12sn Co(co)4 2, 25°c, O. 2MHz/hr Scan Speed, 3 sec. 'l'lme Constant

FIGURE 17

NQR Spectrum of 59co (1/2-J/2) in c12sn Co(co)4 2 25°c,

0.2MHz/hr Scan Speed, 3 sec. Time Constant

69

TABLE XV

Observed NQR frequencies for Cu(I) Complexes

63Cu 65cu S/N(63Cu)

Cu (tu) 2No,t 25,088 MHz 23.280 5/1

Cu(etu) 2Br 32.010 29.620 20/1

Cu(etu)2Cl 'Z?.860 25. 753 50/l

Cu(etu)4 2so4 :u.562 29.250 20/1

b Cu(tu) Cl 22.115 20,40

2 19.296

Cu(etu) 2c104 22.881 3/1

Cu(tu) Br 16,443 3/1 2 16.181 3/1

Cu(dmtu) 3c1 38.804 J6,825 5/1

atu • thiourea; etu = ethylenethiourea.1 dmtu • N,N•dimethylthiourea

bG,L, McKown & E, Swiger, Private Communication.

70

FIGURE 18

NQR Spectrum of 65cu in Cu(e.tu) 2c1, 25°c, 0.05MHz/hr Scan Speed, 1 sec, Time Constant

71

FIGURE 19

NQR Spectrum of 65eu in Cu(etu)4 2so4, 25°c,

O. lMHz/hr Scan Speed, l sec. Time Constant

72

FIGURE 20

NQR Spectrum of 79ar in Cu(etu) 2Br, 25°c,

0,lMHz/hr Scan Time, 1 sec, Time Constant

73

), The range of 5-95MHz was searched using the molybdenum

oxyhalides (Clim.ax Molybdenum Co, - used as received), The compounds

studied and the resonance frequencies found are tabulated in Table

XVII, The intensities of the resonance frequencies are low due to

the low natural abundances of both r-5/2 isotopic species of Mo,

Figure 21 shows the broad Mo resonance in Mooc14 •

Compound

TABLE XVI

Observed NQR Frequencies in Molybdenum Compounds

l - J. 2 2

17. 243 MHz

16.127

Mo

J. - 5. 2 2

36.562 MHz

35.877

Cl

19.)19 MHz

75

FIGURE 21

NQ.Jt Spectrum of a Mo Isotope in MoOC14

25°c, 0.05MHz/hr Scan Time, l sec, Time Constant

DISCUSSION

A, Bis(tetracarbonylcobalt)tin compounds

The obsei-ved resonances are g1 ven in Table XV for 59 Co, which

has a nuclear spin I • ?/2, Both e2Qq and "II were obtained from zz "\, the experimental frequencies by use of the series approximations for

the transition frequencies given in Table II. These values were

further confirmed by using the frequency ratio plot of the type dis-

cussed earlier and shown in Figure 22. The frequency ratios for

't • 0.1 to 0.5 are given in Table III. The asymmetry parameters

for 59co and the frequency ratios as experimentally determined are

given in '!able XVIII.

The occurance of two closely spaced resonances for each

compound indicated two nonequivalent crystallographic sites for the

Co atoms in each. The crystal structure of SiC13co(co)4 is known

The Co atom occupies a site having trigonal (CJv ) point symmetry in

this compound. If we assume that this trigonal environment is retained

in the cobalt atom in the bistetracarbonyl compounds then the symmetry

parameter, 1t. , be equal to zero. The fact that the experimentally

observed 1l values are not equal to zero but have some small values

indicate that the 3-fold symmetry has been distorted slightly, Such

a distortion might be due to non-trigonally symmetric intermolecular

forces in the crystalline solids, intramolecular effects, or crystal

packing eliminating the strict c3v symmetry of the Co sites. If

distortion of the intramolecul.ar bonding caused deviations of the

cobalt sites from c3v SYlllJletry then all inequivalent sites should have

76

0.)0

0.20

0.10

o.oo

77

0,1 0.3 0.5

FIGURE 22

Frequency Ratio vs AsYJftJlletry Parameter Plot for 59co in c12sn Co(C0)4 2

78

TABIE XVII

Experimental Observed Frequency Ratio and the Asymmetry Parameter Determined From F1gure22

Compounds

c12sn [co(co)J 2 1,9565 1.50J6 2. 9420

1.9491 1.5038 2. 9382

Cl~Sn (co(co)4] 2 1.5055

1.4981

{> 2sn [co( CO) 4] 2 1.5008

1.5035

'l

0.065

0.074

0.051

0.089

0.094

0.063

?9

the same 'l values. '!he observed 1l values are different for inequi-

valent sites in the same compound and the differences are of the same

order of 11111.gnl tude as the value of 'l . 'Ibis leads one to conclude

that intermolecular forces rather than intramolecular effects causes

the occurance of multiple resonances in each compound. In effect, the

occurance of the low values of 'l leads to the conclusion that the

cobalt site symmetry can be considered as C)v'

'!here are two methods one can use to discuss the chemical

bonding in these compoundss

l, Compare the experimental para111eters with those of similar

compounds, 'Ibis method serves to point out chemical trends and

substitution effects,

2, Consider the quadrupole coupling constants in terms of the

occupancy of atomic orbitals, This method allows one to formulate the

quadrupole coupling constants in terms of the contribution of electrons

in the different types of bonding orbitals and to vary the electron

denstties in the bonding orbitals to get the best possible agreement

between calculated and observed eoupltng constants, 2 '!able XIX lists the e Q.qzz and 'l values for th~ compounds

studied slong with those of several ra1~te~ compounds, This dat.a can

be useod for a comparison of' the tYJ>(' ,1ust mentioned, The t nducti ve

effect of a Cl atom bonded to a Sn atom will increase the electron

affinity of the empty 4d orbitals of the Sn atom, This will tend to

drain electron density from the filled Jd-orbitals and in turn ~ill

?"Pmove electron density from the CO II' *-orbi ta.ls. The net effect will

be to (a) free some of the a-electron density of the tin atom from the

TABLE XVIII

NQR Ila.ta for 'l'ln Compounds

elqq (59co)a EZ

e~ (l5c1 )a zz

Compound (MHz) (MHz) (Co) (Cl)

Cl2Sn(Co(CO)J 2 146.9 JO.Of 0.070

Cl~ SnfCo(CO)J 2 12?.7 - 0.070

; 2sn[co(C0)4J 2 112.9 0.078

c13snCo(Co)4 163.45 J9.76f o.o ; 3snCo(C0)4 104.11 o.os ClSn {Co(C0)4] 3 l.J5.9 0.09

SnC14 47.7 0.25

Cl2Sn(CHJ) 2 30.8 0.34 Cl2Sn, 2 '35.7! -a. Average for multiple resonances. b. T.L. Br01fll, P.A. l!Hwards, C.B, Harris and J.L. Kirsch, Inorg. Chem.,§., 763 (1969). c. J,D, Graybeal and P,J, Green, J. Phys, Chem,, Zl, 0000 (1969). d, J .D. Graybeal and B.A. Berta, Proceedings 2nd Materials Research Symposium, National Bureau

of Standards, 196?, p. 383. e. P,J. Green and J.D. Graybeal, J. Am, Chem. Soc,, !2.2,, 4305 (1967). f'. Assumed 'L • O. g, D.D. Spencer, J.J. Kirsch and T,L. Brown, J. Inorg, Chem., i, 237 (1970).

Ref,

b CX> 0

b

g

c

d

e

81

Sn-Cl bond and make it more available in the Sn-Co bond, (b) decrease

the net electron population of the Co atom, and (c) strengthen the C:O

bond. These effects will result in (a) an increase of the Co-Sn-Co-bond

angle, (b) an increase in e~zz(Co), and (c) an increase in the C-0

stretching frequency with increased Cl substitution. '!he first and third

points have been substantiated by Patmore and Graham35 while this w<>rk

confirms the second.

2. '!he replacement of a Co(CO\ group by a Cl atom show a

substantial increase in the coupling constant of cobalt atom further

confirming the concept of reduced electron density on the Co atoms due

to halogen inductive effect.

J, The value of e~zz(Cl) increases going from c12sn(Co(Co)4 ) 2

to Snc14 rather than decreases as one might expect if the Cl atom gained

electron density. 'lhis observed change indicates that the net electron

density change on the Sn atoms is relatively small and is insufficient

to provide any net increase of electron density on the Cl atoms in view

of increased competition of the large nwnber of Cl atoms,

4. Substitution of a phenyl group for the Co(Co)4, results in a

increase of e~ (Co) at the remaining cobalt atom, zz 47 BrOlnl has pointed

out that the substitution of a methyl group for a phenyl group has the

same effect on the remaining Co""8.tom. On the basis of the e2qq (Co) zz

values, the Co(co)4 group is a better electron withdrawing group than

either the phenyl or the methyl groups. 'lbe following series, in order

of decreasing electron withdrawing ability, can be established for

compounds of the type studied.

Cl )C Br > Co(C0)4 ) > CH 3

82

'lbe magnitude of the observed coupling constant can be ration-

alized on the basis of a simplified calculation of the EFG tensor

components and the use of an electronically analogous system to estimate

orbital electron populations. '!be molecular field gradient, qzz• can

be expressed in terms of the various type of Jd and 4p electrons by using

the relationship of the field gmdient to angular momentum. In either a

valence bond or molecular orbital approach, qzz arising from the Jd

and 4p electrons can be expressed in terms of atomic orbital populations,

The coupling constant is g1 ven by

e2Qqzz -= eQq320 [N 2 +1/2(Nd +Nd ) -dz x Yz z

(Nd +Nd 2 :) ] + e2Qq410 [ -(N +N ) + N ] x x -v PX Py (58) y .

2 p2

46 A,F. Schreiner has calculated the electron densities for Fe(CO) • 5

'lbese are given in '18.ble XX, 'Ibis is an isolectronic and iaostructural

compound to the bis(tetracarbonylcobalt)tin compounds, By using the

electron densities calculated by Schreiner, hydrogen-like wave functions

and an effective atomic number of Co given by Korol•kov and Makhanek23,

8'.3

TABLE XIX

Orbital Populations in Fe(co)5

Orbit.al Population Orbital Population

Jd 2 1,23 4p z 0,0? z

'.3d:xz • 3d 4p • 4n 0.17 yz x -y

3dxz• 3d x2-y2

4s O,Z?

84

the atomic coupling constant, e2Qq320 , is estimated to be 192 MHz. 'lhe 46 magnitude of ~10 is less than one-fifth that of q320 • 'lhe atomic

2 coupling constant e Qq410 , is estimated to be 12 MHz, 'lhe total coupling

constant as found by using ~uation 58 is 204MHz.

This calculated value is related to the observed value by

2 • (l-R) (e ~zz>calc,

where R is the Steinheimer shielding factor for an open-shell system,

Calculations to date show -0,J < R < 0.2. When onn considers that the

lOlfflr electronegati v!ty of tin, as compared to cnrhon, would probab1y

rAsul t in Nd 2 being largP,r in these compounds as compared to the l"omp lt'!te ly z

? symm.et~ic tY-re, the estim&te of e Qq_zz is re~sonable,

H, r.opner(r) thio11r-ea. an1 s1i"bstH•1t,...r\ tM~urea comiilexes

'fable XVI 11~ts the r-~erved frequencies of the CO!!!p·'.'i'mds

s+.urliP.d. A number of interest1 n17 ohservati ons re~rdinf" tliese observt-d

f.,.~queT1ctes can be made, 1) There is an:preciab1P variation amonp; the

observed f'requenc~es of those compounds which mi~ht be con::>Mered a~

belonging to an isomornhous series. 2) 'Ihe frequencies are in the

viclnity of the reported values for cu2o (26.02 MHz) and KCu(CN) 2

(JJ,468 MHz), 3) There are two absorption frequencies for the bis-

compounds of thiourea with copper halides and one frequency for the other

compounds, 4) The observed absorption frequencies for substituted thio-

urea complexes, in general, are higher than the thiourea complexes. 5)

There is a reversal of the order of the frequencies between the pair,

bis(thiourea)Copper(I) chloride and bromide and the pair, bis(ethylene-

thtourea)Copper(I)chlor1de and bromide,

8.5

2) Since all of the atomic orbitals to be considered fall into

groups having the same principal quantum numbers the radial parts are

common, and the angular and the radial parts are seperable, the radial

part is g1 van by an expression originally developed by Pa.ullng20 ,

- 2 z 3e e

where n • principal quantum number

1 • azimuthal quantum number

A0 • the Bohr radius

(60)

Ze • effective atomic charge • Z-s, s is the screening constant,

and the angular part,

3) The radial contribution is evaluated for Cu(I), which has

an electronic configuration 4s0 3d.10, by using the slater rules given by

Ka.uzmann20 in order to evaluate the necessary screening constant. The

screening constant is calculated to be 25.3, with the effective at~mic

charge being

Ze ~ 29 - 25.3 Q 3.7.

The radial contribution is then

, 2X(J.z)3 x 4,8 x 10-10 14 3 ~--'--) ----·- - ~ 8.56xlO esu cm-e rJ • 43 x (5.3 X 10-9)3 X (1+1)(2+1)

4) The angular part of the atomic were functions are given in

Table XXII, The angular contribution to qrs can be calculated as shown

by the following examples

86

Having enumerated the pertinent features regarding this work

possible explanations will now be considered, 1) The variations that

are observed among compounds such as Cu(etu) 2c1, Cu(etu) 2Br and

Cu(etu) 2No3 are of sufficient magnitude to indicate that there is an

appreciable anion effect operable. This is concluded since the magnitudes

of the differences are greater than normal differences due to non-equivalent

crystallographic sites.

2) The occurance of the observed resonance frequencies in the

vicinity of those of cu2o and KCu(CN) 2, lead one to conclude that the

bonding is probably similiar, Prior work on these compounds by other

investigators indicate predominantely covalent bondin8 in Cu2o and

predominant~ly covalent bonding in the Cu(CN)2- ion of KCu(CN) 2• This

evidence for covalent bonding forms the basis of later discussions of

the bonding,

3) The reason for the occurance of two frequencies for the

bis-compounds is different from that which gave rise to the two closely

Sl>IJ.Ced frequencies which were discussed in the cobalt compounds, For

the copper compounds their appearance is due to the occurance of two

distinctly inequivalent chemical sites and not to intermolecular inter-

actions or crystal pa.eking effects. This point is substantiated by

crystal structure studies on the bis(t~iourea)Copper(I) chloride, 'Ihe +

Cu(tu)2 species form infinite spiral chains with the Cu-CU separations

alternating between a long and a short internuclear distance with

accomp&nying "broad" and "sharp" Cu-S-CU aneles, This alternation of

bond distances along with that of the angles is a strong indication that

the coprer atoms are situation in two different chemical sites. On the

87

basis of the crystal study of the bis(thiourea)chloride and the observation

of two frequencies for each bis-compounds one is lead to conclude that all

of the bis-compounds h&ve structures similar to bis(thiourea)Copper(I)

chloride, 1,e, they form infinite spiral chains with the copper atoms

situated in tetrahedral sites with alternate broad and sharp angles. If

one accepts this conclusion, one would expect two frequencies for the

bis(thiourea)Copper(I) nitrate also. 'Ille experimental result however

shows only one frequency and therefore indicates one chemical site for

the copper atom. 'Ille reason for this is not known. A possible explanation

could be that the size of N03- ion is such that the compound cannot form

the same type structure as the halides and may possibly form a discret

structure similar to the tris(N,N'dimethylthiourea)copper(I) chloride.

4) '!he higher NQR resonance frequencies for the substituted

compounds ca.n be rationalized in terms of the inductive effect of the

substituents on the thiourea ligand. Figure 22 shows the resonance

forms of thiourea, ethylene thiourea and N,N•-dimethylthiourea. It was

pointed out by Dr. Philip Hall in a private discussion, that the o~er

of stability of the resonance form having charge separation are I II

III. On the basis of the resonance forms, one would expect that I will

contribute more electrons to the copper atom to form a complex than

either II or III. Consequently, the copper atom will have the least

p-electron defect if it forms complexes with I. Since the higher the

p-defect, the higher the frequency, the observed frequencies are then

in good agreement with this concept.

5) '!he reversal of the frequencies of the halogen complexes is

difficult to explain on the basts of the electronegativity of chlorine

88

FIGURE 23

Re;onance Forms of Various Ligands

89

or bromine, Since chlorine is more electronegative than bromine, one

would expect the chlorine atoa to withdraw electrons away froa the

copper atoa more than the bromide ion if the Cu-X bond were subtaintally

covalent, Consequently, the coupling constant or the resonance frequency

should be lower for the chlorine compound in both cases, For those

co11pounds whose structures have been determined the Cu-Cl bond length

is such that appreciable ionic character is indicated, An approxbiate

calculation, based on the assumption that all four compounds have the

same structural configuration as the bis(thiourea)copper(I)chloride,

1,e. the copper ato111 is situated at a tetrahedral site, with three

covalently bonded ligands at three corners of the tetrahedron and the

chloride or bromide ion at the fourth corner, shows that the contribution

to the EFG tensor component, q , varies with the internuclear distance zz between Cu and Cl as shown in Figure 23. At a particular internuclear

distance, q goes through a minimum. 'Ibis calculated minimum indicates zz that q can increase with either a decrease or an increasP. of the inter-zz nuclear distance of Cu-X, It is therefore believed that the electro-

negativity of the chlorine or bromine has relative little or no effect

on the reversal of the order of the coupling constants. '!be reversal is

probably due to the particular values of the Cu-X distances 1n the com-

p'>unds. In view of the lack of th'!'! crystal st:ructure data, the above

explanation at it best, a speculation.

Finally, the observed ?9Br and 81Br resonance at JB,828 and

46.588 MHz respectively, for bis(ethylenethiourea)eopper(I) bromide are

worth of mentioned. A simple Townes Dailey calculation using Br2 as a

base, shows that the Cu-Br bond with ?8% ionic character is in line with

the suggestions of AllUll8. and Knobler,

.0

N " I ~ ta H

::s c+

ID ~ c: 0 ..... i 11 ::z en i ::s 0 ct

~

~ :t ~ 0...,

Iii

N

-I .._

06

.0

N

N 0

91

. 6 65 1he pure quadrupole resonance frequencies of Jcu and Cu in

bis(thiourea.)copper(I)chloride were first reported by Sw1ger51 . 'Ihe

crystal structure revealed a pnlymeric chain of alternating copper at.m11s 48

and th1ourea molecules with chloride 1.nterper.::ed. AMma proposed a

dist~rted sp2-hybrjd bond s~~em~ for the Cu-S bonds and an ionic Cl.

!f this scheme is adopted for the his(tu}Cnpper(I}chloride, the bond

directions with respect to an arbitrary x, y, z axis eygtem, (Figure 24)

with +,he Cu-atom at thn or\v,1r. a.re given in Table XXI.

Following the m~thod described on page 28 • and assumtng a

plan~r configuration, the choice hybrid orbitals for Cu-S bonds can be

expressed in the following forms,

'/' l • 0.49358 + 0,8651 Px

4> 2 - 0.61559 - o.4255 Px

41') • 0,6215s - 0,4101 px

- 0.708lp y

+ 0.7055 p • y

These hybrid orbi ta.ls are both normalized and orthogonal. In order to

determine the values for this contribution of a single electron to the

principle z-EFG tensor component the EFG contribution due to one electron

in each atomic orbital must first be evaluated, This is done as followsa

1) The contribution due to one electron in an atomic orbital

described by a wave function, r n. is found by the conventional quantU!ll

mechanical average method,

2 '/' nl• II. ein e drd~

(59)

where fqrsJop is the EFG operation (!able VI.)

s . ...

92

I

....... '.,...... ..• ' --. s,

FIGURE 25

Orientation of Bonds in Cu(tu) 2c1

93

TABLE XX

Bond direction of eu-s and Cu-Cl bonds with respect to x, y, z axis system

x y z

Cu-Sl 90° 17° 107°

eu-s 2 29°29' 120° 95°

Cu-s3 30°19• 119°19• 87°

Cu-Cl 90° 90° 00

TABIE XXI

Angular part of the atomic wave functions

• ( ./3/2 ·hr ) cos e Pz

- /J/41t' sin 9 sin

- ./15/16T (sin 29 Cost/> )

•

• /3/4 Tr sin 9 Cos~ Py

d 2 z

d y

z

- ./5 /16 If ()Cos 2e-1)

• ./15/16"11 (sin 29 sinf' )

q xx

95

~ [3 sin 9 cos e -1J sin e sin ; sin9 ded; __3_1•1.br 2 2 2 2 Px • •

'!able XXIII sU11UD&rizes the angular contributions.

5) 'Ihe tota.l contribution of one electron in a single atomic

orbital is the product of the two individual contributions. 'Ihe values

are tabulated in Table XXIV.

(62)

6) 'Ihe values for the contributions of single electrons in each

hybrid orbital to the principal Z-EFG tensor component are calculated and

given in Table XXV.

7) It is estimated from the electronegativity difference between

the Cu and the S-e.toms that the ionicity of the Cu-S bond is 13.5%. Consequently, the S-atom would contribute 0.87 electron to the Cu-a.tom

if each of the three hybrid orbitals has an equal electron density, In

view of the recent detailed crystal structure analysis done by Amma48 ,

the change density on the Cu- and the S-atoms are estimated and tabulated

in Table XXVI, It was pointed out that one of the sulfur-a.toms forms a

three-center, two -electron-bridge bond with two Cu-a.toms while the other

two sulfur atoms each forms a Cu-S covalent bond with a Cu-atom. If one

assumes equal electron density for these two Cu-S hybrid orbitals, the

charge density on these two hybrid orbitals should be "11 = 0.87 and

l/12 ~ 0,87 electrons respectively. For the three-center, two electron

bridge hybrid, the S-atom must supply both electrons to form the bridge

bond, If this is indeed the case, the charge density of t;3 should be

0.4) electrons.

96

TABIE XXII

Angular contribution of one electron in a single atomic orbital

Atomic orbital q q q qxy qxz q xx yy zz yz

PX -0,8 -+o.4 -+o,4 0 0 0

p -+o.4 -o.a -+o,4 0 0 0 y

p i().4 -+().4 -0,8 0 0 0 z

Atomic orbital

4p y

4p z

TABLE XXtII

'!he total contribution of one electron in a single atomic orbital

-6.84 '3.42

+:3.42 -6.84

+J.42 3.42 -6.84

98

TABLE XXIV

'!he Contribution of a single electron in a Hybrid Orbital to the Z-EFG Tensor Component

Orbital q zz

"' 1

14 -3 2,22 x 10 esu cm

lli. -3 2.33 x 10 esu cm

99

TABLE XXV

Estimated Orbital Populations and Charge Density

No d1T - d...., Bonding D,,. - dr Bonding assumed

6161 0.87 o.87

'/J2 o.87 o.87

'PJ 0,41 0.43

JJ 1 0.5

11 2 0.5 s •cs1) 0.87 O.Y/ 1 •cs )

2 1.74 1.74

'•cs ) o.86 0.36 3

I -(Cl) -1.00 -1.00

j -(Cu) -1.17 -0.67

100

~v ~ 'Ihe (q ) for the Cu-a.tom is then calculated to be 4.94x10 zz -'3 14 -3 esu cm and 4. 08xl0 esu cm. for no d1r - d..- bonding and with d,.. - ~

bonding respectively. 'laking into consideration the shielding effect

for an open shell system, the observed (q )C is therefore expressed as zz u Cov

(q )ob zz

- 4.94 x (1 + 0,2)

14 -3 • J.95 x 10 esu cm

'!'he Sternheimer shielding constant for an open shell system, R00 , for + Cu is not known, The value R .. -0,2 is estimated from the calculated

co

value of the group IA elements. Since cu• is isoelectron with K+ and

(R co )K+ • -o .188, it is therefore reasonable to assume RoJcu + .. -0, 2,

8) We have so far neglected ionic contributions from the

chloride, sulfur and Cu-a.toms, By using the classical electrostatic

espress1on

( ) ionic = qr;z

e(Jcos29-l) r3 (64)

where r is the internuclear distance of Cu-X and 9 is the angle between

the Cu-X bond and the Z""8.Xis (X =Cl, S, Cu), '!he ionic·Z-EFG tensor

components were calculated and are tabulated in Table XXVII, The

observed EFG Z-component due to ions is related to the calculated value

by the Sternheimer shielding constant for a closed shell system, ( 00 , by

l' ;

(q )ionic • (q )ionic zz obs zz cal <1- r > 00

(65)

Cur

Cu II

S +cs > 1

J +cs2)

J •cs3)

J-(Cl)

Total

101

TABLE XXVI

Ionic Contribution of Cl, Cu and S to the qzz-EFG Tensor Component

with d - d assumption

-0,154 x 1014

-0.06 x io14

-0.15 x 1014

-0, 68 x 1014

-0.15 x 1014

-0,42 x 1014

-1,61 x 1014

without d - d assumption

-0.2:1 x io14 esu/cm3

-0.06 x 1014

-0,)4 x io14

-0,68 x 1014

14 -0,)4xJO

-0,42 x io14

-1.99 x 1014

102

The best calculated { 00 value for Cu+ is -17 .o57 • The calculated

nuclear quadrupole coupling constant due to both ionic and covalent

contributions is given by

(e~zz) obs • (e2Qqzz)ionic (1-./J + (e2Qclzr)cov (1-Roo)

-10 -24 14 14 m 4.8xl0 x0.16xlO ((1.2)3.95xl0 -(18)xl.99xl0

6. 627xl0 -'Z?

• -36.o MHz

for no d1J'- d11" bonding and•30.53 MHz for the assumed d-r- d1"' bonding

case, It was also observed that there were two resonance frequencies

for the Cu-a.tom. Following the same procedure, the nuclear quadrupole

coupling constants for the Cu-atom having rCuCl • 3.16 were calculated

to be•J3.6 MHz and•28.03 MHz without d.,,. - d1'"' bonding and with

d1t'- d'fr bonding respectively. The experimentally oooerved values for

the Cu coupling constants are 44.28 and 40.22 MHz. In view of the

uncertainties of both the d.,.- d1Y bonding contribution and the Stern-

heimer effect, the calculated values are in good agreement with the

observed values. One must finally point out that the difference between

the calculated and observed values for two different sites are in excellent

agreement, and indicate that the model used is a reasonable one,

The resonance frequency for both eu63 and 65eu in tris(N•,N-

dimethylthiourea)copper(I)chloride were observed at 38.804 and 36.825 MHz.

The crystal structure has been determined by Amma 48 •49 , Figure 2.5. It

revealed a discreet tetrahedral structure with the Cu atom at the

tetrahedral site. The bond lengths of the Cu-S bonds are the same and 0 they form angles of 112 with the Cu-Cl bond, 'Ihe bond directions in

103

Figure 26

Structure of Bonds in Cu(dmtu)3c1

104

an arbitrary x, y, z axis are given in Table XX VIII, and shown in

Figure 26,

An sp3 hybrid scheme is adopted in evaluating the EFG tensor

com"Ponents at the Cu-a.tom in this system. The four hybrid orbitals

are obtained by em"Ploying the same method as was used for the bis{thiourea)-

Copper(I)chloride and are given by

tifJ l • 0,5 s + 0.866 Pz

'IJ 2 ... 0. 5 s + 0. 66 p - 0. 317 p x z d~ • 0.5s - 0.245 p + 0,707 p - 0.)17 p .... 3 x y z l/J4 - 0,5 s - 0,245 p - 0,707 p - 0,317 p x y z

The values for the contribution of a single electron to the principal

Z-EFG tensor component in each of the hybrid orbitals were calculated as

before and are given in Table xxrx. From the electronega.tivity differences, 'nle ionicity of the

Cu-S bond is estimated to be 13.5% and that of the Cu-Cl bond is

estimated to be 3~. Assuming an equal distribution of electron density

in each of the Cu-hybrid orbitals bonded to sulfur atoms, the charge

densities of the Cu and S-orbitals are estimated and given in Table XXX. cov

The (q ) for the Cu-atom is then calculated to be ' zz cu (-0.94) x io14 esu cm-3. Using the same Sterheimer shieldin~ constant

cov for the Cu-open shell system, the ob~~rverl (qzz)cu is e~~?"Pssed,

(q )Cov = ( ) Cov ( ) zz obs qzz cal 1 - Roo

14 -1 ~ 1.2 x (-0.04) ~ JO esu cm ·

.. -1 1.., , 014- "" ~ J •. ) x ,, .. Sil, cm

10.5

TABLE XXVII

Bond Direction of Cu-Sand Cu-Cl Bonds With Respect to x, y, z Axis System

x y z

eu-s1 98.54 109.5? 112°

Cu-52 98°.541 109° .571 112°

Cu-SJ 98°.541 109°.571 112.0

Cu-Cl 900 90° 00

106

FIGURE~

Orientation of Cu(dmtu)3c1

107

TABLE XXVIII

The Contribution of a Sin~le Electron in a Hybrid -orbital to the Z-EFG Tensor Component

Bond (orbital)

Cu-s1 (I/I 2)

cu-s2 (I/' 3)

eu-s3 ( , 4)

Cu-Cl ('11)

d yz d xy

0,7 x 1014 esu/cm'3

1. 24 x 1014

1.24 x io14

6 14 - .13 x 10 J4

2.32 x 10

14 -1.16 x 10

-1.16 x io14

108

TABLE xxrx Est1mated Orbital Populations and Charge Densities

No - d1r -dfJ'" bonding with cir - d'1'" bonding

'/' 1 0.87 0.87

.,,, 2 0,87 o.87

"' 3 0,87 o.87

'1'4 0.70 0,70

J •cs1) o.87 O.J?

J+(S ) 2

0.87 0.37

J +(s3) o.~7 -+o.J7

J-(Cl) -0.3 -0,3

J-(Cu) -2.31 -1.31

71'1 0,5

1i 2 0.5

109

Again, the ionic contr1bution of the sulfur and chlorine atoms

must be considered. The ionic contributions to q)zz are calculated by

using the same procedure used for Cu(tu)2c1 and are t~bulated in Table

XXXI.

The observed EFG-Z components due to ions is rel~ted to the

calculated value as followss ionic

Q.zz)obs ionic {

= qcal (l- a> )

= 18 x ( -0.764)

a -13.75 x 1014 esu/cm3 14 J For no d Tt - d 11"' and -7. 9 x 10 esu/cm for d,,. - d11' respectively. The

nuclear quadrupole coupling constant due to both ionic and covalent

contributions is 2 ionic cov

=e Qq ( qobs + qobs )

• 4.8 xlOlO x 0.16 x lo-24 (-13.71 6.6'2:1 x 10-'Z'l

·. • -17 .'2:1 MHz

- 1.1'3 )

for no d11' -d'Tt' and -10.45 NH~ for dw - dn respectively. These estimated

values are considerable lon~r than the experimentally observed value,

77.6 MHz. The reason for the difference is not yet known. Further in-

formation regarding the details of the crystal structure is needed

before any conclusions can be drawn.

C) Molybedenum Oxyhalides 95 Table XVII tabulated the observed frequencies for Mo or

Mo97 a.lonp; with the obst..:::·:cd. c135 frcc;,uencies for Mooc14 and Moo2c12.

For one of the Mo ~.&o<.opes, both of which have a nuclear spin, I=5/2,

J •cs2)

S •cs ) 3

A-(Cl)

d xy

d yz

d 2 z

Total

110

TABLE XXX

Ionic Contribution of Cl, S and Cu to q ) zz

w1 th no -d -d

14 3 -0 .186 x 10 esu/cm

14 3 -0.186 x 10 esu/cm

-0.186 x 1014 esu/cm3

14 1 -0,204 x 10 esu/cm

-f4,64 x 1014

-2.32 x 1014

14 .+4.64 x 10

-o. 764 x io14

with -d -d

-0,079 x 1014 esu/cm3

-0.079 x 1014

-0. 079 x 1014

-0,204 x 1014

+J,48 x 1014

-1.74 x 1014

14 -l.74xl0

-4, 64 x 1014

14 i4.64 x 10

-o.441 x 1014

111 2 the values of (e Qq)zz and ~were obtained from the experimental frequen-

cies by u~e of the series approximation for the transition frequencies 54 .

given in Table II. NMR studies hav~ shown the ~tic of the moments ,

Mo95; Mo97 to be equal to 9.J. From this ratio and the observed Mo fre-

quencies, one should expect to observe the other pair of frequencies

at either approximately 300 MHz or 3 MHz, both of which are beyond the

operating range of the spectrometer, The: assignment of the observed

frequencies to a particular isotopic species is therefore impossible

at the present time, We have also investigated a number of other Mo-

compound and were unable to observe any resonances, The limited number

of observation severely restrict the discussion of any relationships

of the observed frequencies to the bonding properties. We have, how-

ever opened up an interesting field for further studies,

·'

1.

2.

3.

4,

5. 6.

7.

8,

9.

10.

11.

12.

13.

14.

112

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The vita has been removed from the scanned document

ABSTRACT

The work described in this dissertation represents an effort to

extend the application of Nuclear Quadrupole Resonance spectroscopy to

the study of transition element compounds. Using a conventional noise

controlled superregenerative spectrometer compounds of cobalt, copper

and molybdenum have been investigated.

Three biscobalt(tetracarbonyl) tin(II) compounds were investigated

and the 59co resonances measured in each. Each compound exhibited a

doublet indica.Uve of two crystallographic inequvilent sites. The asymm.e-

try parameters were all between 0.005 and 0.10 indicating little distor-

tion of the cobalt enviroments from the expected C3v SYlllJl\etry. The

coupling constants as obtained by use of a series approximation for

the transition frequencies and confirmed by a frequency ratio plot were

Clz Sn (Co(C0)412 -146.9MHz, (C6H5)ClSn (co(C0)4)2 -137.? MHz,

(C6H5) 2sn(Co(C0)4]2 -112.9 MHz. The observed coupling constants correlate

with the inductive effects of the substitutents in the tin.

The study of several Copper(!) coordination compounds represents

the first known attempt at using Cu nuclear quadrupo1e coupling constants

to study ond.1ng in a situation other than an isolated compound. Aasum-

ing zero asymmetry parameters the following 63cu couplin~ constants

were observed; Cu(tu) 2Nor ':i0.18 MHz, Cu(tu) 2c1 - 41.41 MHz, Cu(tu)2Br-

J2.62 MHz, Cu(etu)2c104- 45.76 MHz, [cu(etu)4J 2 SOq.- 63.12 MH~,

Cu(etu)2c1 - 55.72 MHz, Cu(etu)2Br- 64.02 MHz, Cu(dll!tu) 3c1 - 7?.60MHz~

The ligands used were tu-thiourea, etu-ethylene thiourea, and dmtu•

N,N• dimethylthiourea. 'nle crystal structures of only Cu(tu) 2c1 and

Cu(dmtu)3Cl are known making direct comparison difficult. The general

increase of the coupling constants with ligand substitution correlates

w1 th the partial charge on the sulfur atom of the free ligand. The re-

versal of the order of the coupling constants between the thiourea and

ethylen thioure& halides indicates an an appreciable ion contribution

to the coupling constant from the halogen. The obserim.tion of ?9Br

resonance at JB.83 MHz in Cu(etu)2Br also confirms this point. By

using sp2 and sp3 hybridization schemes for Cu(tu)2c1 and Cu(dmtu)jll

the cou-pling constants were calculated to be 36.0MHz and 17.2? MHz

respectively. This represents resonable agreement in view of the uncer-

tainties in the Sternheimer factor used and the approximate nature of

the model. The allowance for~ - dtr bonding between the Cu and S ato111s

decreases the calculated constants indicating that such bonding probably

is of little importance.

Resonance were observed for Mo isotope in both ~ooc14 and Mo02C12.

Both possible Mo resonances as well as the CJ.resonances were observed.

ThP. particular 1sotope to which the ,..esona.nces bPlong 1s as vet undeter•

l!li ned since tho"3e belonging t-:> the other Mo tsoto,,es w~ th. T~ 1 will