progress in inorganic chemistry · 2013. 7. 23. · progress in inorganic chemistry edited by...

PROGRESS IN INORGANIC CHEMISTRY

Edited by

STEPHEN J. LIPPARD

DEPARTMENT OF CHEMISTRY MASSACHUSETTS INSTITUTE OF TECHNOLOGY CAMBRIDGE, MASSACHUSETTS

VOLUME 32

AN INTERSCIENCE@ PUBLICATION JOHN WILEY & SONS New York Chichester Brisbane Toronto Singapore

Progress in Inorganic Chemistry

Volume 32

Advisory Board

THEODORE L. BROWN

JAMES P. COLLMAN

F. ALBERT COTTON

RONALD J. GILLESPIE

RICHARD H. HOLM

GEOFFREY WILKINSON

UNIVERSITY OF ILLINOIS, URBANA, ILLINOIS

STANFORD UNIVERSITY, STANFORD, CALIFORNIA

TEXAS A & M UNIVERSITY, COLLEGE STATION, TEXAS

McMASTER UNIVERSITY, HAMILTON, ONTARIO, CANADA

HARVARD UNIVERSITY, CAMBRIDGE, MASSACHUSETTS

IMPERIAL COLLEGE OF SCIENCE AND TECHNOLOGY, LONDON, ENGLAND

PROGRESS IN INORGANIC CHEMISTRY

Edited by

STEPHEN J. LIPPARD

DEPARTMENT OF CHEMISTRY MASSACHUSETTS INSTITUTE OF TECHNOLOGY CAMBRIDGE, MASSACHUSETTS

VOLUME 32

AN INTERSCIENCE@ PUBLICATION JOHN WILEY & SONS New York Chichester Brisbane Toronto Singapore

An Interscience" Publication

CopyrightQ 1984 by John Wiley & Sons, Inc.

All rights reserved. Published simultaneously in Canada.

Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley & Sons, Inc.

Library of Congress Catalog Card Number: 59- 13035 ISBN 0-471-87994-0

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

Contents

The Stereodynamics of Metal Complexes of Sulfur-, Selenium-, and Tellurium-Containing Ligands ............................................ 1

By EDWARD W. ABEL and KEITH G. ORRELL Department of Chemistry, University of Exeter Exeter, England and SURESH K. BHARGAVA Department of Chemistry, I.P. College Bulandshahr, India

Five-Coordinated Structures. ............................................. 1 19 By ROBERT R. HOLMES Department of Chemistry, University of Massachusetts Amherst, Massachusetts

Homo- and Heteronuclear Cluster Compounds of Gold .................. 237 By KEVIN P. HALL and D. MICHAEL P. MINGOS Inorganic Chemistry Laboratory, University of Oxford Oxford, England

Electrides, Negatively Charged Metal Ions, and Related Phenomena .... 327 By JAMES L. DYE Department of Chemistry, Michigan State University East Lansing, Michigan

Long-Range Electron Transfer in Peptides and Proteins.. ................ 443 By STEPHAN S. ISIED Department of Chemistry, Rutgers, The State University of New Jersey New Brunswick, New Jersey

The Polyhedral Metallaboranes Part I.

By JOHN D. KENNEDY Department of Inorganic and Structural Chemistry, University of Leea's Leeds, England

Metallaborane Clusters with Seven Vertices and Fewer ......... 519

Subject Index ............................................................ 681 Cumulative Index, Volumes 1-32.. ...................................... 707

The Stereodynamics of Metal Complexes of Sulfur.. Selenium.. and Tellurium-Containing Ligands

EDWARD W . ABEL. SURESH K . BHARGAVA. P and KEITH G . ORRELL

Department of Chemistry University of Exeter Exeter. England

CONTENTS

I . INTRODUCTION AND SCOPE OF REVIEW . . . . . . . . . . . . . 2

I1 . EXPERIMENTAL TECHNIQUES . . . . . . . . . . . . . . . . . 5

A . Exchanging NMR Spin Systems . . . . . . . . . . . . . . . . 6

C . Static NMR Parameters . . . . . . . . . . . . . . . . . . . 10 D . Experimental Procedures . . . . . . . . . . . . . . . . . . . 12 E . Activation Parameters . . . . . . . . . . . . . . . . . . . . 13 F . Nonstationary State NMR Methods . . . . . . . . . . . . . . . 14

111 . PYRAMIDAL ATOMIC INVERSION . . . . . . . . . . . . . . . . 15

B . Theoretical Bandshape Analysis . . . . . . . . . . . . . . . . 7

A . Introduction . . . . . . . . . . . . . . . . . . . . . . . 15 B . Mechanisms of Inversion . . . . . . . . . . . . . . . . . . . 16

1 . Intramolecular Rearrangement via a Planar Intermediate . . . . . . . 16 2 . Dissociation-Recombination . . . . . . . . . . . . . . . . 17 3 . Bimolecular Exchange . . . . . . . . . . . . . . . . . . 18 4 . Miscellaneous Chemical Reactions . . . . . . . . . . . . . . 18

C . Atomic Inversion and Bond Rotation . . . . . . . . . . . . . . . 18 D . Stereochemistry and Inversion . . . . . . . . . . . . . . . . . 20

1 . Inversion at a Chiral Center . . . . . . . . . . . . . . . . 20 2 . Inversion at a Prochiral Center . . . . . . . . . . . . . . . 21 3 . Inversion at an Achiral Center . . . . . . . . . . . . . . . 22

E . Theoretical Calculations of Inversion Energies . . . . . . . . . . . . 23 1 . Ab Initio Molecular Orbital Calculations . . . . . . . . . . . . 23 2 . Semiempirical Calculations . . . . . . . . . . . . . . . . . 24

TDr . Bhargava's present address is: Department of Chemistry. I.P. College. Bulandshahr- 203001. India .

2 EDWARD W . ABEL. SURESH K . BHARGAVA. AND KEITH G . ORRELL

F . Inversion at Sulfur. Selenium. and Tellurium . . . . . . . . . . . . 24 1 . Chalcogen Hydrides . . . . . . . . . . . . . . . . . . . 24 2 . Organochalcogen Compounds . . . . . . . . . . . . . . . . 25

G . Inversion at Sulfur and Selenium in Main Group Complexes . . . . . . . 28 H . Inversion at Sulfur. Selenium. and Tellurium in Transition Metal Complexes 28

1 . Titanium. Zirconium. and Hafnium . . . . . . . . . . . . . . 29 2 . Vanadium. Niobium. and Tantalum . . . . . . . . . . . . . . 29 3 . Chromium. Molybdenum. and Tungsten . . . . . . . . . . . . 29 4 . Manganese. Technetium. and Rhenium . . . . . . . . . . . . . 32 5 . Iron. Ruthenium. and Osmium . . . . . . . . . . . . . . . 36 6 . Cobalt. Rhodium. and Iridium . . . . . . . . . . . . . . . 38 7 . Nickel. Palladium. and Platinum . . . . . . . . . . . . . . . 40 8 . Copper. Silver. and Gold . . . . . . . . . . . . . . . . . 52

I . Factors Influencing Atomic-Inversion Energies . . . . . . . . . . . . 53

2 . Nature of the Metal Center . . . . . . . . . . . . . . . . . 54 3 . n-Conjugation Effects in the Ligands . . . . . . . . . . . . . 56 4 . Ligand Ring Strain Effects . . . . . . . . . . . . . . . . . 58 5 . The Trans Influence upon Inversion Energies . . . . . . . . . . . 60

1 . Nature of the Inverting Center . . . . . . . . . . . . . . . 53

IV . FLUXIONAL REARRANGEMENTS . . . . . . . . . . . . . . . . 61

A . Cyclic Ligand Complexes . . . . . . . . . . . . . . . . . . 62 B . Open Chain Ligand Complexes . . . . . . . . . . . . . . . . . 70 C . Bridging Ligand Complexes . . . . . . . . . . . . . . . . . . 77

2 . Methyl Scrambling . . . . . . . . . . . . . . . . . . . 81 3 . Ligand Pivoting . . . . . . . . . . . . . . . . . . . . 84

D . Chelate Ligand Complexes . . . . . . . . . . . . . . . . . . 91

1 . Ligand Switching . . . . . . . . . . . . . . . . . . . . 77

V . CONFORMATIONAL CHANGES OF CHALCOGEN HETEROCYCLIC RINGS IN COORDINATION COMPLEXES . . . . . . . . . . . . . . . . . 101

A . Five-Membered Rings . . . . . . . . . . . . . . . . . . . . 101 B . Six-Membered Rings . . . . . . . . . . . . . . . . . . . . 103

1 . Conformational Changes of Six-Membered Ring Ligands . . . . . . . 103 2 . Conformational Changes of Ring Systems in Dimetallic Bridged Systems 105

C . Polychalcogen Metallocycles . . . . . . . . . . . . . . . . . 106 D . Chalcogen-Bridged Ferrocenophanes . . . . . . . . . . . . . . . 109 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 111 References . . . . . . . . . . . . . . . . . . . . . . . . . 111

I . INTRODUCTION AND SCOPE OF REVIEW

Although metal complexes of coordinated sulfur ligands have been known for nearly a century (74) little was clearly established about their structures and bonding until comparatively recently . The whole field of metal complexes of group VIB ligands was reviewed in 1965 (185) and. more recently (in 1981).

THE STEREODYNAMICS OF METAL COMPLEXES OF SULFUR LIGANDS 3

the specific area of transition metal complexes of thioethers, selenoethers, and telluroethers was comprehensively surveyed (207).

Early x-ray diffraction studies (79, 249) clearly illustrated the pyramidal environment of the coordinated chalcogen atoms. In all the complexes described in the above reviews it is therefore safe to assume that the chalcogen atoms are approximately pyramidal with bond angles about sulfur, selenium, or tellurium approximately tetrahedral, and the single electron lone pair in an essentially pure sp3 orbital. Distortions from ideal tetrahedral angles are to be expected, and have indeed been found in cases where the metal and chalcogen atoms are incorporated in a ring. The potentiality also exists, in all cases, for the coordinated chalcogen to undergo an atomic inversion analogous to that of nitrogen.

Pyramidal inversion had been suggested as early as 1924 (197) and confirmed experimentally for nitrogen atoms some years later (51, 109). However, it soon became apparent both as a result of experimental studies and theoretical calculations that on moving from first-row to second- or third-row atoms the energy barrier to inversion dramatically increases. The high configurational stability of sulfur in such species as sulfonium ions (103) and sulfoxides (218) is therefore not unexpected. However, as more data were accumulated and the general factors governing energies of pyramidal inversion became established, it became apparent that if group VIB atoms were coordinated to metals with d orbitals available for n back bonding, then the inversion barriers could be significantly reduced and, in the case of the chalcogens S, Se, and Te configurationally nonrigid species would arise.

The first recognition of this type of complex was the sulfur chelate complex [PtCl2(MeSCH2CH2SMe)1 (20). Variable temperature NMR studies were able to detect the varying rates of inversion of the coordinated sulfurs, thereby illus- trating how the effects of (p-d)n overlap of the S and Pt orbitals could greatly accelerate the pyramidal inversion of such coordinated atoms compared to their uncoordinated counterparts. This observation further indicated that the inversion energy had been brought within the range of NMR detection, namely around 20-100 kJ mole-'.

By the mid-1960s NMR was well established as a most valuable spectroscopic technique for studying relatively high-energy internal molecular motions, such as restricted bond rotations, ring conformational changes, as well as nitrogen pyramidal inversions (220, 155). Such phenomena were very clearly detected by variable temperature NMR studies but their associated energy barriers were not easily assessed with accuracy. Early calculations based on band coalescence measurements were fraught with errors arising primarily from the unreliable absorption band shapes and inadequate sample temperature control and measurement. This uncertainty meant that many of the early estimates of nitrogen inversion barriers and barriers associated with other dynamic processes had subsequently to be revised in the light of improvements both in the methodology

4 EDWARD W. ABEL, SURESH K. BHARGAVA, AND KEITH G . ORRELL

of NMR band shape analysis and in the reliability of the NMR experimental technique.

The development of the theory underlying total NMR band shape analysis, culminating in the powerful general-purpose computer programs for dynamic spin problems (67, 239), has greatly contributed to the improved reliability of energy barrier data currently deduced from variable temperature studies. The important development of Fourier transform methods and the consequent mul- tinuclear capabilities of the NMR technique have also greatly aided the study of dynamic processes since spectra can now be more reliably and rapidly obtained than previously, and one or more types of nuclei can often be chosen as sensors for a given dynamic process.

This review discusses the currently important role of dynamic NMR spectroscopy for studying internal molecular motions and illustrates its key role in revealing the inversion characteristics of individual metal-coordinated S , Se, and Te atoms. When complexes involve ligands with two or more chalcogen atoms there exists the possibility that the inversion process can be followed by a switching of coordination from one chalcogen atom to another. In the case of dinuclear or polynuclear metal complexes there is the further possibility that the inversion process will lead to a switching of allegiance of chalcogen lone pairs from one metal atom to another, in other words the usual (intrametallic) inversion about a single metal atom may be transformed into an intermetallic switching of the whole ligand moiety. The review discusses such phenomena in the case of sulfur and selenium atoms, and, furthermore, describes how the intermediate species of such metal-switching processes appear to be highly nonrigid species which lead to further intra- and intermolecular fluxional phenomena.

When the coordinating atom is incorporated in a ring, then both ring conformational and pyramidal inversion processes can occur. The rates of these phenomena may be very similar, as in the case of nitrogen atoms incorporated in six-membered heterocyclic rings, and this ambiguity has led to considerable controversy in interpreting the stereodynamics of such systems. This review examines whether similar difficulties arise in the case of S, Se, and Te heterocyclic ring complexes.

In summary, the review describes how, in the past decade in particular, dynamic methods have been invaluable in detecting pyramidal inversion of S , Se, and Te atoms, and how such studies have been precursors to a rich variety of other fluxional phenomena when such atoms are coordinated to transition metals. The review is organized into three major sections. After a general dis- cussion of dynamic NMR methods, there follows a major section on pyramidal inversion. This section includes a fairly exhaustive review of the literature on inversion studies of S, Se, and Te atoms in main group and, particularly, in transition metal complexes. The next major section is concerned with the variety


of fluxional rearrangements which may be initiated by pyramidal inversion in di-, tri-, and polychalcogen metal complexes. Finally, the conformational changes associated with metal-coordinated chalcogen heterocyclic rings and polychalcogen metallocycles are described.

II. EXPERIMENTAL TECHNIQUES

The three most widely used physical techniques for measuring energies of pyramidal inversion and other internal molecular rearrangements are vibrational (infrared and Raman) spectroscopy, microwave spectroscopy, and NMR spectroscopy. In the context of this review the techniques differ primarily in their “interaction times” or “timescales. ” This parameter, which is essentially the reciprocal of the frequency of electromagnetic radiation utilized by that technique, is a measure of the response time of that spectral technique to molecular movement. For vibrational spectra, the interaction times are in the range

sec; for microwave spectra they are approximately lO-’O sec, and for NMR spectra in the range 10-1-10-9 sec, which implies that vibrational spectroscopy is a “fast” technique in terms of nonrigidity of molecules, whereas NMR is a “slow” technique. The nature of the quantized energy on which the spectral technique is based, together with the interaction time of that technique, collec- tively determine how the spectra of that particular technique are sensitive to molecular motion, be it in terms of the number of detailed signals/bands, their associated frequency, their intensity, and/or their shape.

For practical purposes, energy barriers in the approximate range 0-20 kJ mole-’ are measurable by microwave techniques, energies greater than approximately 20 kJ mole-’ by vibrational spectra and energies in the approximate range 35-100 kJ mole-’ by NMR methods. The relative ‘‘slowness’’ of the NMR technique makes it particularly sensitive to a whole range of relatively slow, high-energy motions in molecules.

NMR also differs from the other mentioned spectral techniques in that it has always used coherent radiation as a light source, in contrast to the thermal light sources of the other techniques. This peculiarity provides the distinctive possibility of quantitatively studying dynamic processes at a molecular level, even though experimental measurements are performed on macroscopic samples in thermodynamic equilibrium. The random motions of molecules in fluids act to remove the phase relationship which exists between the coherent radiation and the quantum mechanical state function of the microscopic systems. Spectroscopic techniques concerned with this interplay are “dynamic” techniques (191) and it is for this reason that the terminology of “dynamic NMR” (DNMR) first suggested in 1968 (68) has become common parlance.

6 EDWARD W. ABEL, SURESH K. BHARGAVA, AND KEITH G . OFtRELL

A. Exchanging NMR Spin Systems

The phenomena of exchange broadening and coalescence of NMR lines are widely known throughout chemistry, and the observation of such phenomena can provide much information at a qualitative level about the dynamic process(es) occurring. This section is concerned with the accurate quantitative assessment of energy barriers from a rigorous analysis of the band shape changes which accompany internal molecular motions. We differentiate between the many types of dynamic processes detectable by NMR and show how all these seemingly diverse processes can be classified under a single general scheme of spin labeling. This is a necessary and important preliminary to discussing the development of a general theory of NMR band shape analysis.

The f is t broad classification of exchange processes is into intermolecular and intramolecular exchanges. The DNMR technique is capable of handling both types of process in principle. Important intermolecular exchange processes include proton exchanges between acidic molecules, dissociation of covalent compounds into ions and the reverse process of ion combination, and ligand exchanges involving organometallic compounds or inorganic complexes. The last mentioned is particularly relevant to this review, where, as will be seen later, the majority of main group and transition metal complexes of chalcogen ligands undergo some type of ligand dissociationlrecombination process at elevated temperatures. The theoretical dynamic study of such systems can be carried out using the same computational treatment as for intramolecular rate processes, providing that there is no indirect spin-spin coupling occurring between nuclei. If there is such coupling then a different theoretical framework has to be adopted (24 1, 183). This theory has not been generalized to the same extent as the theory for int-amolecular exchanges, and the authors are not aware of any general computer program currently available.

The study of internal mobility of molecules and the resultant intramolecular site exchanges of magnetic nuclei has been the primary concern of DNMR to date. When the dynamic process in question simply produces a permutation of the nuclear sites the process is termed a topomerisation (69). Many fluxional movements in organometallic species fall into this category. Enantiomerisations produce the same type of change, providing the compounds are investigated in common (i.e., achiral) solvents. The detection of enantiomeric changes by NMR, however, depends on the presence of prochiral groups (134) (e.g., -CX,Y) in the molecule. The pairs of identical nuclei in such groups are diastereotopic (201) and during enantiomerisations their positions are mutually exchanged. In diastereoisomerisations and constitutional isomerisations, however, each nuclear spin takes up a new magnetic environment. Thus, symmetry considerations can often allow a distinction to be made between different possible mechanisms of a process.


The exploitation of prochiral groups for studying stereodynamics is, however, far wider than its use in enantiomerisations. Such groups have been extensively used in studying restricted bond rotations, pyramidal inversions, and ring conformational changes in general ( 153). This review discusses their widespread use as probes of such processes in metal complexes of coordinated chalcogen ligands.

B. Theoretical Bandshape Analysis

The effects of site exchanges of magnetic nuclei can be incorporated into either a classical or quantum mechanical theory of NMR bandshapes. The classical approach based on an extension of the phenomenological Bloch equations has the merits of straightforward mathematical description and visual immediacy of the factors affecting bandshapes. However, such an approach suffers from the severe limitation of being applicable only to molecules where strong nuclear spin-spin coupling is absent. The classical approach has been reviewed on nu- merous occasions and here the reader is simply referred to the particularly explicit treatment by Sutherland (240) and the more concise mathematical description by Binsch (66). The classical theory is well suited to handle two-site exchanges where the populations and effective spin-spin relaxation times of the nuclei at the two sites are either equal or unequal. The theory has been generalized to multisite exchange by Kubo and Sack (175, 228) using matrix methods, but as will be seen more efficient mathematical methods are now available for handling such systems. The classical approach can also be used for weakly spin-coupled systems, that is, systems where IJ,I << Iu, - u,l. In this case each line of the spectrum is considered as a “site” with the appropriate fractional population being determined by the splitting pattern. However, any spin problem involving coupled nuclei is far more efficiently handled by a quantum mechanical treatment.

Bloch’s phenomenological equations describe the NMR effect for an ensemble of mutually independent nuclear spins. Such an approach can clearly not be applied to strongly coupled spins since it would be meaningless to consider transitions of individual nuclei when the strong scalar coupling interactions lead to a mixing of energy levels. All quantum mechanical NMR bandshape calculations are based on a density matrix treatment first developed by Kaplan (159) and Alexander (39). Several reviews of this approach have appeared (155, 187, 83, 238, 241), but we summarize here the treatment by Binsch (68, 67, 169, 66, 70).

In density matrix notation the equation of motion for nuclear spins in a magnetic field is given by the Liouville equation

dgldt = -i[fk,(i] (1)

8 EDWARD W. ABEL, SURESH K. BHARGAVA, AND KEITH G . ORRJZLL

where fi is the density matrix describing the system, and % is the usual spin Hamiltonian. Binsch used Eq. 1 as the starting point for the development of a unified theory of exchange-broadened NMR line shapes. Following the Liouville representation of quantum mechanics the matrix elements of p are the components of a vector p in a multidimensional space called Liouville space. Each state of the system is then represented by one state vector. If the system consists of identical molecules described by identical Hamiltonians the state vector and Liouville space are described as “primitive. ” If, however, the total system consists of several exchanging subsystems then it is described by a “composite” state vector in a composite Liouville space. The Hamiltonian operator in Eq. 1 can then be replaced by the Liouville operator i which includes a time-independent part i, and a time-dependent part ii. Lo consists of both nuclear Zeeman and spin-spin scalar coupling interactions. Binsch then introduced the relaxation operator R which is normally proportional to TT-’, Tf being the effective transverse relaxation time of the nuclei, and includes the field inhomogeneity con- tribution such that

1 1 ?As, - +- _ - _

Tf T2 2

Finally, the exchange operator X was introduced. This operator contained the first-order rate constants for exchange between the various subsystems, its precise form depending on whether the exchange is intra- or intermolecular.

Equation 1 can now be written as

For steady-state resonance conditions, dyildt = 0 and thus we have

0,fi = -ia (4)

where 0, now includes only the nuclear Zeeman and spin-coupling operators Lo, and vector a possesses components proportional to the fractional populations of individual subsystems.

The total transverse complex magnetization, M*, can be calculated from the expectation values of the nuclear spin lowering operators I - . The imaginary part of M* gives the absorption line shape, v, where

v = Im(M*) = Im(-iI- - 0;’ a) ( 5 )

This equation could be used as written for bandshape computations, but in order to avoid the time-consuming inversion of the matrix 0,, Binsch introduced


the similarity transformation

where D is the resulting diagonal matrix. D can be expressed as a sum of a radiofrequency independent part A and a unit matrix E such that

It follows that

In other words, inversion of the matrix 0, has been reduced to the straightforward inversion of the diagonal matrix D. Further substitutions lead to the final bandshape equation in the form

v = - cRe(Q-S) (9)

where the elements of the vectors Q and S are given by

sj = c I;u,u,-'P, i,k

where Pi = u / c and c is a proportionality constant. It is of great practical value that the vectors Q and S have simple physical interpretations. The imaginary part of the spectral vector Q gives the line positions (in rad sec-I) while the real part represents the line widths. The real part of the shape vector S determines the intensities of the lines and the imaginary part represents their deviation from Lorentzian shape. Thus, in the absence of exchange Im(Si) = 0, Re(Si) = intensity of the ith line, Im(Qi) = 2nvi and Re(Qi) = Tfi - ' . For nonzero exchange rates, Im(Si) # 0, and the other parameters vary with the exchange rate until in the fast exchange limit Im(Si) approaches zero and the number of chemical shift values v i associated with Im(Q,) is reduced.

Binsch and Kleier (7 1) programmed the above theory into a general bandshape program which was later improved to include symmetry and magnetic equiva- lence factoring (168). The program, as originally written, is capable of handling systems of up to four nuclear spins where there is no symmetry, and up to six spins where symmetry factorization is possible. However, various modifications and extensions of the program have been made by others, including the present authors. The program requires as input data information concerning (1) number of nuclei, ( 2 ) number of chemical configurations between which exchange may

10 EDWARD W. ABEL, SURESH K. BHARGAVA, AND KEITH G . O M L L

occur, (3) symmetries of such configurations, (4) chemical shifts and coupling constants for each configuration, (5) populations of each configuration, (6) Tf values (Eq. 2) of the nuclei in the absence of exchange, and (7) first-order rate constants for exchange between all specified configurations. Theoretical spectra based on these static parameters are then synthesized and subsequently compared visually with the experimental spectra. Any of the above parameters (but more usually, the rate constant values) can then be varied systematically until a good visual fit between theoretical and experimental spectra is obtained.

Visual fitting of total line shapes is a sensitive but somewhat subjective procedure. It works well for relatively complex line shape changes, where individual spectra contain lines of varying degrees of exchange broadening. It is scientifically preferable, however, if the optimum fitting of theoretical and experimental spectra is calculated within the program by some iterative scheme. This scheme, however, requires the digitization of the raw spectral data with the ability to allow for nonideal spectral features such as baseline increment, baseline drift, and spurious signals. The analysis program DNMR5 has recently been developed (239) with the above capabilities, but is not yet widely used, first because few laboratories have the necessary spectral digitization facilities, and second because the iterative procedures in the program may occasionally fail unless high-quality experimental spectra (i.e., those free from chemical impurity bands, residual solvent signals, spinning sidebands, etc.) or efficient spectral editing facilities are available. Nevertheless, this iterative approach is very likely to become the accepted method in the future as modem NMR spectrometers become linked to or incorporate more powerful data-processing facilities.

The majority of the energies discussed in this review have been obtained by the visual fittings of total bandshapes or by the very approximate method of measuring the rate constant for the temperature (T,) at which band coalescence occurs, using the relationship

T A U k(T,) = -

v2

This equation, however, only strictly applies to simple two-site exchange in the absence of spin coupling. Both methods require knowledge of certain static parameters for the nuclei, and the accurate measurement of these parameters is now outlined.

C. Static NMR Parameters

One of the key factors limiting the accuracy of bandshape analyses is the accuracy of the static NMR parameters for the nuclear spin system, namely the


chemical shifts and spin-coupling constants of all nuclei involved in the rate process, the populations of each nuclear site, and the effective transverse relaxation times, T f , of the nuclei in the absence of exchange. In the strict NMR sense the latter is, of course, a dynamic parameter, but need not be treated as such in the present context.

Accurate values of all these parameters are usually required for each temperature at which a bandshape fitting is performed. Unfortunately, all these parameters are normally temperature dependeh to varying extents. Their values can usually be measured directly from first-order spectra or by iterative analysis of complex second-order spectra at temperatures below those producing appre- ciable line exchange broadening. Any significant temperature dependences of these values must be taken into account by appropriate extrapolation procedures in order to deduce their magnitudes at the temperatures at which bandshape analyses are carried out.

The temperature dependencies of chemical shifts and coupling constants may or may not be linear. Site populations are expected to follow a van't Hoff plot of In K' against T - I , where Ke is the standard equilibrium constant for the i F j site equilibrium and is given by the ratio of the site populations, p, /pi . Equilibrium constants for pairs of nuclear spin systems may alternatively be expressed in terms of the differences in the standard Gibbs free energy, enthalpy, and entropy functions.

The determination of the T ; parameter, which may be obtained from the natural line width in the absence of exchange, A u , , ~ by the relationship

often presents the most difficulty. An accurate assessment of this parameter is most important for temperatures where the exchange broadening is not extensive, that is, toward the high and low temperature extremes of study. This parameter includes a field inhomogeneity factor (see Eq. 2) which makes it essential that good field homogeneity be maintained at all temperatures to ensure that the difference between Tf values for the slow and fast exchange limits is very small. In such cases a linear interpolation for the intermediate temperatures is feasible. Caution must be exercised in measuring T f values in the low temperature range where increases in solvent viscosity and changes in molecular correlation times may prsduce abnormally broad lines (i.e., abnormally short T f values).

Further details concerning the pitfalls of measuring static NMR parameters have been published elsewhere (240, 66, 70).

12 EDWARD W. ABEL, SURESH K. BHARGAVA, AND KEITH G. ORRELL

D. Experimental Procedures

If the aim is to obtain reliable quantitative information about internal molecular motions, then considerable care must be exercised in the basic experimental NMR procedures. The following five conditions need to be quite rigorously satisfied.

1. Spectra recorded under CW conditions must be free from transient phenomena and therefore relatively slow sweep rates must be employed. With the FT technique this difficulty does not arise. Instead, baseline artifacts and signal distortion caused by pulse breakthrough and nonlinear phase effects can occur, and must be avoided or minimized whenever possible.

Signal saturation effects must be avoided if the bandshape theory (Section 1I.B) is to be applied. Thus, relatively low-amplitude B 1 fields must be employed in the CW method, particularly when narrow frequency ranges are being ex- amined. With the FT technique signal distortions due to saturation effects do not arise, but nuclear responses to a series of RF pulses will be diminished if the pulse interval is too short compared to the nuclear T , values.

The homogeneity of the external magnetic field should be as high as p,ossible at all temperatures in order that the absorption lines approximate to Lorentzian shape, and any small spin-spin splitting effects are revealed as clearly as possible. It is also important from the point of view of nuclear Tf values (Section 1I.C) that the field inhomogeneity line broadening be minimized at all temperatures.

Relaxation broadening effects due to quadrupolar nuclei such as 14N and *D should be absent. This problem can normally be avoided by simultaneous decoupling of these nuclei.

The temperature control of the NMR experiment should be optimized. It is of crucial importance that during any DNMR experiment the sample be maintained in a series of highly uniform and stable temperature environments which can be accurately measured. The temperature range accessible on most modem spectrometers is around - 150 to 150°C. The precise measurement of NMR sample temperatures has produced a diversity of recommended procedures in the literature. However, two procedures are commonly adopted. The first is a substitution method, which involves an NMR tube, partially filled with a suitable solvent and containing a thermocouple, being exchanged for the sample tube immediately before and after recording spectra. The second method uses an NMR thermometer liquid (e.g., methanol or ethylene glycol for 'H studies) in a sealed capillary within the NMR tube. Temperatures are based on measured internal chemical shifts for the thermometer liquid (127). Both procedures should be capable of temperature measurements of at least k 0.5"C accuracy.

2.

3.

4.

5 .


Further details on experimental procedures in DNMR are contained in a general NMR text (194) and two specific DNMR monographs (150, 230).

E. Activation Parameters

Measurements of ‘ ‘best-fit’’ rate constants for nuclear exchange processes as a function of temperature enable barrier energies of the defined rate process to be calculated according to the Arrhenius or Eyring theories, using the well- known relations

k = A exp( -2) k = K(?) exp(*)

where the symbols have their usual significance. The Eyring approach is to be preferred on the grounds that its activation parameters afford a more precise chemical interpretation for nuclear exchange processes than do the Arrhenius parameters. Using the thermodynamic relation

AGt = AHS - TAS’ (17)

in combination with the logarithmic form of Eq. 16, one obtains

from which the activation parameters, AGt, AH’ and AS* can be obtained from the data of a full variable temperature bandshape analysis. If NMR fittings are made solely at the coalescence temperature, T,, then only an approximate value of AGS(T,) may be evaluated.

The question of the errors associated with the Eyring parameters requires careful consideration (66). The accuracy of the “best-fit” rate constants depends on many factors relating to the characteristics of the spectra being fitted in addition to purely experimental considerations. Of particular importance is the fact that the values will be least reliable at temperatures furthest from the coalescence temperature(s), and therefore a weighted least-squares fitting procedure is necessary. Since the errors are based on logarithmic plots, however, a linearized error treatment is strictly inadequate and only a full error propagation analysis is appropriate (66). This procedure, however, is rarely adopted by most

14 EDWARD W. ABEL, SURESH K. BHARGAVA, AND KEITH G. ORFSLL

workers and, indeed, the majority of energies quoted in this review are based on simple linearized least-squares fittings. It is well established that energies quoted in terms of AG* values are least prone to systematic error and therefore this parameter is normally used for comparison purposes. A proper error analysis (70) shows that the RMS error of AG' is approximately given by

u(AG*) (u(AHS) - Tu(AS*)( (19)

and typical values of u(AGt) are in the range 0.1-0.5 kJ mole-'. Highest accuracy in bandshape analyses is achieved when spectra exhibit

relatively gross and complex bandshape changes over a wide temperature range. This behavior influences one's choice of sensor nuclei when such an option exists. 'H DNMR studies are still the most common but protons suffer from the considerable disadvantage of a restricted resonance frequency range. Most other nuclei do not have this limitation but may instead suffer from low intrinsic sensitivity. I3C is such a nucleus. DNMR analysis with this nucleus can, however, often suceed where 'H analysis fails on account of small internal chemical shifts. It must be recognized, however, that too wide a chemical shift dispersion of a nucleus (as is often the case with nuclei such as "Se and '9sPt) may lead to such excessive line broadening in the coalescence region that the absorption becomes virtually undetectable, thus precluding any fittings precisely where highest accuracy should normally be expected.

In the context of this review the nuclei that have been most studied are 'H, I3C, 77Se, 'I9Sn, '*'Te and 195Pt. These are all I = nuclei and are relatively easy to detect. It is unfortunate from the point of view of the many sulfur- coordinated ligand complexes discussed later that sulfur does not possess a suitable NMR isotope, 33S (I = 3) being highly quadrupolar and of very low receptivity relative to that of protons (DH = 1.71 X lo-').

F. Nonstationary State NMR Methods

All the foregoing discussions on DNMR methods have referred to stationary- state conditions. Nowadays, with pulse FT methods being routinely available, many nonstationary methods of magnetization transfer offer themselves as alternative techniques for studying motional processes. These are based primarily on double resonance (122, 143) or spin echo methods (221). One particular advantage of nonstationary methods is that they often allow rate constants for an exchange process to be reliably measured at temperatures below those where line broadening is significant. Another major advantage lies in their potential ability to provide mechanistic information on nuclear exchange processes, in contrast, in most instances, to bandshape analysis.

Spin saturation transfer experiments are a particularly important type of non-


stationary NMR method. Using modem pulse equipment such experiments are quite easy to perform and can be designed either selectively to saturate certain spectral lines by decoupling or selectively to invert certain lines by applying specific 180" pulses. Both experiments are potentially very important DNMR techniques. The saturation transfer experiment has recently been successfully applied to the molybdenum complex [Mo(u-Cp)(q-Cp)(NO)(S2CNR,)I where a clear distinction between possible 1,2- and 1,3-fluxional shifts around the u- Cp ring was made (145).

Selective inversion of spectral lines can be very effectively achieved by applying the DANTE pulse sequence (205). This sequence utilizes pulse trains of precisely defined characteristics for the nuclei requiring inversion, followed by a nonselective observing pulse. The technique has recently been used to determine the mechanism of correlated rotations in triaryl derivatives of phosphorus and arsenic (247). A combination of bandshape fittings and selective population inversion studies confirmed the exclusive operation of one mechanism, out of a total of seven differentiable mechanisms. Since the spectra involved were second order in nature, a density matrix approach was developed to account for the observed changes in the DANTE spectra.

Such a combination of bandshape analyses of exchange broadened spectra with selective population inversion studies, yielding both energetic and mechanistic information on internal rate processes, is likely to be very important in the future development of DNMR.

It has also been shown recently that two-dimensional NMR studies can reveal similar mechanistic information about nuclear exchange processes (152), and further developments of this technique are also likely.

There have been no reports to date of any of the above techniques being applied to chalcogen coordinated metal complexes, however, but they would appear to provide potentially important ways of investigating the fluxionality of metal-chalcogen bonds where various mechanisms may occur (Chapter IV).

111. PYRAMIDAL ATOMIC INVERSION

A. Introduction

The mode of molecular vibration known as inversion exists, in principle, for all nonplanar molecular species. However, the term is normally restricted to the spontaneous inversion of configuration of an atom bonded to three substituents in a pyramidal geometry and possessing one lone pair of electrons (1). The inversion vibration usually involves the interchange of two energetically equivalent configurations via a planar transition state (2).


1 2 1

The two equivalent configurations are referred to as invertomers. Atomic inversion is a particularly subtle molecular rearrangement since only a reversal of configuration occurs, no actual chemical bonds necessarily being broken nor any second chemical reactant being involved in the vast majority of cases. However, a number of alternative inversion mechanisms to the one shown above, have been shown to exist and these are now briefly outlined.

A number of general reviews (80, 182, 161, 215, 180) and a textbook (184) on pyramidal inversion in molecules have appeared.

B. Mechanisms of Inversion

1. Intramolecular Rearrangement via a Planar Intermediate

Classically, a vibrational mode leads to inversion through a coplanar arrange- ment of groups about the inverting center. The observed rate of inversion depends on the height of the twofold potential barrier corresponding to the planar configuration. The height and shape of a potential barrier determine the precise way in which the energy of the vibrational mode is quantized. However, in quantum mechanical terminology there exists the possibility of an alternative mechanism of inversion, a mechanism that does not require traversal over the twofold barrier.

It is a well-known result of quantum mechanics that if a particle resides in a potential well, the walls of which are of finite height and thickness, then the wave functional description of that particle provides a finite probability for the particle to exist on the other side of the wall. Thus, in the case of the double- wall potential for inversion there is a finite probability that the molecule can tunnel through the barrier separating one invertomer from another. The rate of tunnelling depends in a complex way on the mass and energy of the molecule, and on the exact shape of the twofold barrier profile.

For finite barriers each vibrational level is split into symmetric and antisym- metric sublevels. For infinite barriers these sublevel pairs are degenerate, whereas for very small barriers, each vibrational level is either symmetric ( +) or anti- symmetric (-) and therefore in both cases no splitting occurs. In the case of finite barriers, the inversion splitting of the vibrational levels gives rise to the detection of microwave absorption (e.g., for NH3, O+ - 0- = 0.79 cm-’) when there is no change in vibrational quantum number, and splitting (“dou-

THE STERODYNAMICS OF METAL COMPLEXES OF SULFUR LIGANDS 17

bling") of near infrared absorption signals (e.g., for NH3, 0' - 1' = 950 cm-') when the vibrational quantum number increases by unity. Thus, both types of spectroscopy may be applied to study the inversion properties of symmetric top XY3 molecules. The accuracy of the barrier measurements does, however, depend on the correctness of the potential function used to describe the inversion vibration. There have been many attempts to establish the most satisfactory form of expression, Gaussian, quadratic-quartic, and parabolic functions being applied with varying degrees of success.

When the inverting molecules have lower symmetry then C3", (i.e., asymmetric tops) their microwave and infrared spectra become highly complex due to the more complicated set of vibration-rotational energy levels associated with such species and the many allowed transitions between them. This complexity limits the usefulness of these techniques for less symmetrical molecules. Such a limitation, however, does not extend to the NMR technique, which in contrast is only applicable to the study of inversion in molecules that are asymmetric tops, and where the process involves a change of chemical environment of at least one of the attached groups.

The distinction between classical and nonclassical inversion mechanisms is not important when only barrier energies are being considered. For the compounds discussed in this review, however, the classical description is more appropriate since tunneling is only thought to be the dominant mechanism for atoms with very light substitutent atoms (e.g., H or D), for relatively low barriers, and for cases when the invertomers are not diastereoisomers. Virtually all cases of inverting group VIB atoms in chalcogen complexes discussed later do not fall into these categories.

2 . Dissociation-Recombination

Inversion of configuration can result if one (or more) of the three substituents on the central atom becomes detached and subsequently recombines. Such a process, (3 4 3), however, is not strictly a pyramidal inversion.

3 4 3

In cases where the covalent bonds are of reasonable strength such a mechanism can be neglected. In virtually all examples of transition metal-coordinated chalcogen atoms, the metal-E bonds are relatively strong, and an intramolecular


inversion is thought to occur. If the metal and chalcogen atoms have suitable magnetic isotopes, then the detection of mutual nuclear spin-spin coupling throughout the temperature range over which inversion occurs provides conclu- sive evidence for a purely intramolecular rearrangment.

The high-energy inversions of sulfoxide species (218, 199) are examples of dissociation-recombination processes, with the bonds in these cases being broken homolytically .

3. Bimolecular Exchange

A possible inversion mechanism, but one that has been clearly established in only a few cases, is that of bimolecular encounters.

5 6

Such a mechanism (5 F= 6) is likely to be very sensitive to substrate concen- tration, and to the degree of aggregation in solution. It has been shown to occur in some cases of phosphorus inversion and in a number of organometallic compounds containing partially ionic M-C bonds, but to date it has not been postulated as the dominant mechanism for inversion of group VIB atoms.

4 . Miscellaneous Chemical Reactions

There have been a number of reports of inversion of configuration occumng as a result of intra- and intermolecular chemical reactions. All the cases referred to are examples of high-energy inversion processes. Sulfoxides have been particularly studied (199) but these will be discussed in more detail later.

C. Atomic Inversion and Bond Rotation

The possibility of bond rotation accompanying, or even being synchronous with, an atomic inversion process must always be borne in mind with open- chain ligands, because, although the two processes are intrinsically different, either process may produce tiie same spectral changes. The problem may be illustrated by the hypothetical species Me2E1E&R1R2 in Scheme 1. E, is an inverting atom such as N, S , or Se, whereas E, is a noninverting atom (on the NMR time scale) such as P. An interchange of methyl environments of either the syn or anti species can be effected either by a slow, rate-limiting El inversion


Scheme 1

followed by a El-& bond rotation or by a slow rotation followed by a rapid El inversion. Thus, if the variable temperature NMR spectra show the expected change from two methyl signals to one, there is no assurance that the barrier energy computed is due to inversion rather than E,-Q restricted rotation. The NMR spectra will be sensitive only to the rate-detennining (i.e., higher energy) process and once that is rapid in NMR terms then the spectra will be unaffecfed ,by the other process. This is a general feature of NMR spectra of simple two- site exchanges (but not of more complex exchanging spin systems) and illustrates the limitations of such simple spin systems.

In cases where observed spectral changes may be due to inversion or rotation, additional experiments are usually required to decide between them. For example, bulky groups on the central atom usually slightly decrease the inversion barrier but increase the rotation barrier. Conjugative groups tend to accelerate pyramidal inversion but have little effect on rotation. Changes of solvent may affect the two processes differently, particularly if the pyramidal inversion proceeds via a bond dissociation mechanism.

When the inverting atom is incorporated in a rigid three-, four-, or five- membered ring or in a bicyclic framework, the rotational interconversion is eliminated. Incorporation in a six-membered ring is no answer to the problem, however, because chair-to-chair ring reversal may compete with the heterocyclic atom inversion. An example of such a case would be a metal-coordinated thian ring (Scheme 2). Interconversion of the equatorial M-S species (i), (iii) and the axial species (ii), (iv) can occur either by sulfur inversion or ring reversal and the 'H NMR spectral changes of the above system can therefore be due to either process. Without additional evidence from solvent, steric, or conjugative effects no clear interpretation is, in general, possible. The above example is to some extent artificial, however, since it was possible in this case to prove conclusively that both processes were occurring, albeit at different rates (15).

20 EDWARD W. ABEL, SURESH K. BHARGAVA, AND KEITH G. ORRELL

Ring rev 1 11 Ring rev

Scheme 2

This proof was achieved by considering the total structuE of the conformational isomers of the complexes concerned, namely [MX,(S(CH,),),] (M = Pd(II), Pt(II), X = C1, Br). The sulfur inversion energy was shown to be significantly higher than the ring reversal energy. An analogous result was found for the dihalogen complexes of 1 ,4-dithian (146).

Unambiguous distinction between pyramidal inversions and bond rotations involving nitrogen atoms has been particularly difficult to achieve in a number of cases (180), but with group VIB atoms, the distinction is rather more well defined.

D. Stereochemistry and Inversion

An aspect of paramount importance in studying pyramidal inversion is the stereochemistry of the inverting center, which of course depends on the nature of the substituents attached to the inverting atom. The following three cases need to be considered.

1. Inversion at a Chiral Center

The product of pyramidal inversion at a chiral center (7 8) will be the enantiomer of the parent molecule as can be seen below.

I 8

However, if one of the groups X, Y, or Z possesses a chiral substituent, inversion


will then produce an invertomer which is diastereotopic with the starting molecule, that is, they are not related by any symmetry operation.

The presence of prochiral substituents (153) on a chiral inverting atom has provided a valuable means of studying pyramidal inversion by proton NMR spectroscopy.

The carbon atom in this molecule is a prochiral center and the pair of attached methylene protons are diastereotopic and anisochronous. The product of this inversion (9 10) is an enantiomer of the original molecule. The methylene protons are still diastereotopic and anisochronous but their magnetic environments have been interchanged. Thus, an increased rate of inversion at E will lead to a coalescence of the AB quartet of the methylene protons, and thence to an averaged singlet. The dynamic spin problem is therefore AB

This situation has been widely exploited for studying S or Se inversion in metal complexes. A particularly suitable R group is Me,Si since it imposes no scalar H-H couplings on the methylene protons and its methyl absorption is well removed from the methylene absorption.

BA.

2. Inversion at a Prochiral Center

Prochirality of an inverting center gives rise to two possible situations. Firstly, there is the case of a chiral assembly attached to a prochiral inverting center. This produces an enantiomer of the starting molecule as shown in 11 12.

11 12


Secondly, there is the case where the inverting center is prochiral by virtue of being attached to two identical groups, each of which possesses a prochiral atom.

13 14

The two -CH2R groups in the molecule are enantiomerically related both before and after the inversion (13 14). However, the geminal protons Ha and Hb in each group are diastereotopic and anisochronous and they preserve these features after inversion. This is an example, therefore, of two enantiomerk pairs of diastereotopic protons. The net result of inversion at E is to interchange the H, and Hb environments and thus the dynamic spin system for the methylene protons is [AB], [BA],. In the absence of any scalar coupling between protons in different -CH,R groups, the spin problem simplifies to AB BA. This stereochemistry has been utilized in a number of cases for studying inversion at metal-coordinated sulfur and selenium atoms. It'should be noted that in such a situation rotation about the E-C bonds will not produce the interchange of the H, and H b proton environments and can therefore be dispensed with as an explanation of the spectral changes.

3. Inversion at an Achiral Center

Inversion at an atomic center with three identical attached atoms or 'groups (15 e 16) produces a degenerate, superimposable species which only differs from the starting molecule by a permutation of the identical groups X.

15 16

This, therefore, represents a topomerisation process (69), but in this instance, is not detectable by NMR since the chemical shift of the X nuclei is unaffected by the inversion process. Thus, the NMR technique would be unable to detect NH3 inversion even if its rate came within the NMR time scale. The same reasoning applies to all pyramidal molecules of type AH3. Inversion energy

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