chapter 7-1 chemistry 481, spring 2015, la tech instructor: dr. upali siriwardane e-mail:...

Chapter 7-1Chemistry 481, Spring 2015, LA Tech

Instructor: Dr. Upali Siriwardane

e-mail: [email protected]

Office: CTH 311 Phone 257-4941

Office Hours:

M,W 8:00-9:00 & 11:00-12:00 am;

Tu,Th, F 9:30 - 11:30 a.m.

April 7 , 2015: Test 1 (Chapters 1, 2, 3)

April 30, 2015: Test 2 (Chapters 6 & 7)

May 19, 2015: Test 3 (Chapters. 19 & 20)

May 19, Make Up: Comprehensive covering all Chapters

Chemistry 481(01) Spring 2015


Chapter 7. An introduction to coordination compounds

The language of coordination chemistry 7.1 Representative ligands7.2 Nomenclature

Constitution and geometry7.3 Low coordination numbers 7.4 Intermediate coordination numbers 7.53Higher coordination numbers 7.6 Polymetallic complexes

Isomerism and chirality7.7 Square-planar complexes 7.8 Tetrahedral complexes 7.9 Trigonal-bipyrmidal and square-pyramidal complexes7.10 Octahedral complexes7.11 Ligand chirality


Chapter 7. An introduction to coordination compounds

Thermodynamics of complex formation

7.12 Formation constants7.13 Trends in successive formation constants7.14 Chelate and macrocyclic effects7.15 Steric effects and electron delocalization


Coordination compoundA compound formed from a Lewis acid and Lewis

base.A metal or metal ion acting Lewis acid (being an

electron pair acceptor) and a atom or group of atoms with lone electron pairs Lewis base electron pair donor forms an adduct with dative or coordinative covalent bonds.

Ni(ClO4)2 (aq)+ 6NH3 → [Ni(NH3)6](ClO4)2 (aq)

The Lewis bases attached to the metal ion in such compounds are called ligands.


The coordination number (CN) CN of a metal ion in a complex is defined as the

number of ligand donor atoms to which the metal is directly bonded.

[Co(NH3)5Cl]2+

CN is 6, 1 chloride and 5 ammonia ligands each donating an electron pair.

For organometallic compounds. An alternative definition of CN would be the number of electron pairs arising from the ligand donor atoms to which the metal is directly bonded.


1) What is a coordination compound?


Coordination sphere

• Coordination sphere - the sphere around the central ion made up of the ligands directly attached to it. Primary and secondary coordination sphere.


Preparation of Complexes

• The figure at left shows cyanide ions (in the form of KCN), being added to an aq. solution of FeSO4.

• Since water is a Lewis base, the Fe2+ ions were originally in the complex [Fe(H2O)6]2+

• The CN- ions are driving out the H2O molecules in this substitution reaction that form the hexacyanoferrate(II) ion, [Fe(CN)6]4- .

[Fe(H2O)6]2+

+ 6 CN- [Fe(CN)6]

4- + 6 H2O


Various Colors of d-Metal Complexes

The color of the complex depends on the identity of the

ligands as well as of the metal..

Impressive changes of color often accompany substitution

reactions.


Structures and symmetries

• Six-coordinate complexes are almost all octahedral (a).

• Four-coordinate complexes can be tetrahedral (b) or square planar (c).

• (Square planar usually occurs with d8 electron configurations, such as in Pt2+ and Au3+.)


Representing Octahedral Shapes• Instead of a perspective drawing (a), we can

represent octahedral complexes by a simplified drawing that emphasizes the geometry of the bonds (b).


LigandsThe Brønsted bases or Lewis base attached to the

metal ion in such compounds are called ligands.

These may be

Simple ions such as Cl–, CN–

Small molecules such as H2O or NH3,

Larger molecules such as H2NCH2CH2NH2

N(CH2CH2NH2)3

Macromolecules, EDTA and biological molecules such as proteins.


Representative Ligands and NomenclatureBidentate Ligands

Polydentate Ligands• Some ligands can simultaneously occupy more

than one binding site.• Ethylenediamine (above) has a nitrogen lone pair

at each end, making it bidentate. It is widely used and abbreviated “en”, as in [Co(en)3]3+.


Ethylenediaminetetraacetate Ion (EDTA)• EDTA4- is another example of a

chelating agent. It is hexadentate.

• This ligand forms complexes with many metal ions, including Pb2+, and is used to treat lead poisoning.

• Unfortunately, it also removes Ca2+ and Fe2+ along with the lead.

• Chelating agents are common in nature.


Porphyrins and phthalocyanins


Chelates• The metal ion in [Co(en)3]3+ lies

at the center of the three ligands as though pinched by three molecular claws. It is an example of a chelate,

• A complex containing one or more ligands that form a ring of atoms that includes the central metal atom.


Naming Transition Metal Complexes• Cation name first then anion name.• List first the ligands, then the central atom• The ligand names are made to end in -O if negative• Anion part of the complex ends in -ate

Eg. Cu(CN)64- is called the hexacyanocuprate(II) ion

• The ligands are named in alphabetic order• Number of each kind of ligand by Greek prefix• The oxidation state of the central metal atom

shown in parenthesis after metal name• Briding is shown with ( -oxo)


Some Common Ligand Names


Names of Ligands (continued)


Coordination Sphere Nomenclature• Cationic coordination sphere

• -ium ending

Anionic coordination sphere• -ate ending


Examples• [Co(NH3)4Cl2]Cl:

• dichlorotetramminecobalt(III) chloride• [Pt(NH3)3Cl]2[PtCl4]:

di(monochlorotriammineplatinum(II)) tetrachloroplatinate(II).

• K3[Fe(ox)(ONO)4] :

• potassium tetranitritooxalatoferrate(III)


Use bis and tris for di and trifor chelating ligands• [Co(en)3](NO3)2 :

• tris(ethylenediamine)cobalt(II) nitrate • [Ir(H2O)2(en)2]Cl3

• bis(ethylenediamine)diaquairidium(III) chloride

• [Ni(en)3]3[MnO4] :

• Tris(ethylenediamine)nickel(II) tetraoxomanganate(II)


Naming

• [Cu(NH3)4]SO4

tetraaminecopper(II) sulfate• [Ti(H2O)6][CoCl6]

hexaaquatitanium(III) hexachlorocobaltate(III)

K3[Fe(CN)6]

• potassium hexacyanoferrate(III)


2) Give the formula of following coordination compounds

a)Dichlorobis(ethylenediammine)nickle

b) Potasium trichloro(ethylene)platinate(1-)


c) Tetrakis(pyridine)platinum(2+) tetrachloroplatinate(2-)

d) Tetraamminebis(ethylenediamine)

--hydroxo- -amidodicobalt(4+) chloride


3) Give the names of following coordination compounds

a) [Co(NH3)6]Cl3;

b) trans-[Cr(NH3)4(NO2)2]+ ;

c) K[Cu(CN)2] ;

d) cis-[PtCl2(NH3)2] ;

e) fac-[Co(NO2)3(NH3)3]Cl3


The Eta(h) System of Nomenclature

• For for p bonded ligands number of atoms attached to the metal atom is shown by hn

(h5 -cyclopentadienyl) tricarbonyl manganese

tetracarbonyl (h3

-allyl) manganese, Mn(C3H5)(CO)4


Isomers

• Both structural and stereoisomers are found.• The two ions shown below differ only in the

positions of the Cl- ligand, but they are distinct species, with different physical and chemical properties.


4) What is the geometry and coordination number of compounds in the problem above?

a) [Co(NH3)6]Cl3;

b) trans-[Cr(NH3)4(NO2)2]+ ;

c) K[Cu(CN)2] ;

d) cis-[PtCl2(NH3)2] ;

e) fac-[Co(NO2)3(NH3)3]Cl3


5) Draw the formula and find the BITE of following ligands.

a) 2,2'-bipyridine (bipy) ;

b) terpy;

c) cyclam;

d) edta;


Ionization Isomers• These differ by the exchange of a ligand with an

anion (or neutral molecule) outside the coordination sphere. [CoSO4(NH3)5]Br has the Br- as an accompanying anion (not a ligand) and [CoBr(NH3)5]SO4 has Br - as a ligand and SO4

2-as accompanying anion.


Ionization IsomersThe red-violet solution of [Co(NH3)5Br]SO4

(left) has no rxn

w/ Ag+

ions, but forms a ppt.

when Ba2+

ions are added.

The dark red solution of [CoSO4(NH3)5]Br

(right) forms a ppt. w/ Ag+

ions, but does not

react w/ Ba2+

ions.


Hydrate Isomers• These differ by an ex-change

between an H2O molecule and another ligand in the coordination sphere.

• The solid, CrCl3. 6H2O, may be any of three compounds.

• [Cr(H2O)6]Cl3 (violet)

• CrCl(H2O)5]Cl2.H2O (blue-green)

• CrCl2 (H2O)4Cl.2H2O (green)

• Primary and secondary coordination spheres


Linkage IsomersThe triatomic ligand is the isothiocyanato, NCS-. In (b) it is the thiocyanato, SCN-. Other ligands capable or forming linkage isomers are

NO2- vs. ONO -

CN - vs. NC - .(a) NSC

- ligand (the N is closest to the center);

(b) SCN- ligand (S is closest the center)


Coordination Isomers

• These occur when one or more ligands are exchanged between a cationic complex and an anionic complex.

• An example is the pair [Cr(NH3)6][Fe(CN)6] and[Fe(NH3)6][Cr(CN)6].


Stereoisomers• Ionization, hydrate, linkage, and coordination

isomers are all structural isomers. • In stereoisomers, the formulas are the same. The

atoms have the same partners in the coordination sphere, but the arrangement of the ligands in space differs.

• The cis- and trans- geometric isomers shown in next slide differ only in the way the ligands are arranged in space.

• There can be geometric isomers for octahedral and square planar complexes, but not for tetrahedral complexes.


Square Planar ComplexesGeometric Isomers• Properties of geometric isomers can vary greatly. • The cis- isomer below is pale orange-yellow, has a

solubility of 0.252 g/100 g water, and is used for chemotherapy treatment.

• The trans- isomer is dark yellow, has a solu-bility of 0.037 g/100 g water, and shows no hemotherapeutic effect.


6) Describe the geometrical isomerism in following compounds:

a) [Co(NH3)4Cl2]+ ;

b) [IrCl3(PPh3)3] ;

c) [Cr(en)2Cl2] ;


cis and trans-PtCl2(NH3)2


Trans Effect & Influence


Preparation Geometrical Isomers


Optical IsomerismThe two complexes at left are mirror images.

(The gray rectangle represents a mirror,

through which we see somewhat darkly.)

No matter how the complexes are rotated,

neither can be superimposed on the other.

Note only four of the six ligands are different.


Combined Stereoisomerisms• Both geometrical and optical isomerism can

occur in the same complex, as below. The trans- isomer is green.

• The two cis- isomers, which are optical isomers of each other, are violet.


Identifying Optical IsomerismIf a molecule or ion belong to a point group with a Sn axis is not optically active


Molecular Polarity and Chirality Polarity

• Polarity:Only molecules belonging to the point groups Cn, Cnv and Cs are polar. The dipole moment lies along the symmetry axis formolecules

belonging to the point groups Cn and Cnv. • Any of D groups, T, O and I groups will not be

polar


ChiralityOnly molecules lacking a Sn axis can be chiral.

This includes mirror planes

and a center of inversion as

S2=s , S1=I and Dn groups.

Not Chiral: Dnh, Dnd,Td and Oh.


Optical Activity


Reactions of Metal ComplexesFormation constants– the chelate effect– Irving William Series– Lability


7) Pick the chiral compounds among the following:

a) [Co(en)3]3+ ;

b) cis-[Cr(en)2Cl2] ;

c) c) trans-[Cr(en)2Cl2] ;


Formation of Coordination Complexestypically coordination compounds are more labile or

fluxional than other molecules X is leaving group and Y is entering group

MX + Y MY + X

One example is the competition of a ligand, L for a coordination site with a solvent molecule such as H2O

[Co(OH2)6]2+ + Cl- [Co(OH2)5Cl]+ + H2O


Formation Constants

Consider formation as a series of formation equilibria:

Summarized as:


Typically: Kn>Kn+1

Expected statistically, fewer coordination sites

available to form MLn+1

eg sequential formation of Ni(NH3)n(OH2)6-n 2+

Values of Kn


Breaking the RulesOrder is reversed when some electronic or chemical

change drives formation

Fe(bipy)2(OH2)22+ + bipy Fe(bipy)3

2+

jump from a high spin to low spin complex

Fe(bipy)2(OH2)2 t2g4eg2 high spin

Fe(bipy)3 t2g6 low spin


Irving William Series

Values of log Kf for 2+ ions including transition metal species Lewis acidity (acceptance of e-) increases across the per. table, thus forming more and more stable complexes for the same ligand system

Kf series for transition metals:

Mn2+< Fe2+ < Co2+ < Ni2+ < Cu2+ >Zn2+


Irving William Series


Bonding and electronic structure

Bonding Theories of Transition Metal Complexes

• Valance Bond Theory

• Crystal Field Theory

• Ligand Field Theory or Molecular Orbital Theory


Valance Bond Theory

”Outer orbital" (sp3d2) and ”Inner orbital" (d2sp3)

[CoF6]3- - Co3+ : d6

[Co(NH3)6]3+ - Co3+ : d6


Spectrochemical Series for Ligands

• It is possible to arrange representative ligands in an order of increasing field strength called the spectrochemical series:

I¯ < Br¯ < -SCN¯ < Cl¯ < F¯ < OH¯ < C2O42¯ < H2O < -

NCS¯ < py < NH3 < en < bipy < o-phen < NO2¯ < CN¯ < CO


8) Use valence bond theory (VBT) to predict the electron configurations, the type of bonding (Inner and outer orbital) and number of unpaired electrons in following compounds:

a) [Co(CN)6]3- ;

b) [CoCl6]3-;

c) [Fe(NH3)6]3+;


Crystal Field Theory

• In the electrical fields created by ligands

• The orbitals are split into two groups: a

set consisting of dxy, dxz, and dyz stabilized

by 2/5Do, known by their symmetry

• classification as the t2g set, and a set

consisting of the dx2-y2 and dz2, known as

the eg set, destabilized by 3/5Do where Do

is the gap between the two sets.


Crystal Field Splitting of d Orbitals


Octahedral Crystal Field Splitting


9) What are the symmetry labels of s,p, and d orbitals in tetrahedral (NiCO)4) and square-planar ([PtCl4]2-) and octahedral (Cr(CO)6) compounds.


10) Explain the effect of ligands on the d orbitals in octahedral, tetrahedral, trigonal-bipyramid and square-planar coordination compounds using Crystal Field Theory.

Octahedral,

Tetrahedral,

Trigonal-bipyramid

Square-planar


11) [Ti(H2O)6]3+ shows a absorption at 20300 cm-1. Absorption values for similar coordination compounds of Ti3+ with different ligands are given below. Based on their absorption values arrange the following ligands in a Spectrochemical Series.

Absorption(cm-1)

Ligand H2O CN- PPh3 F- NH3

20300 20500 20455 20100 20400


Crystal Field Stabilization Energy

• Crystal Field stabilization parameter Do


Crystal Field Stabilization Energy

d7 case.

Weak field case

The configurations would be written t2g5 eg

2

5(-2/5Do) + 2(+3/5Do) = -4/5Do

Strong field case

The configurations would be written t2g6 eg

1

6(-2/5Do) + 1(+3/5Do) = -9/5Do


CFSE & Paring Energy

[Fe(H2O)6]2+. Iron has a d6 configuration, the value of

Do is 10,400 cm-1 and the pairing energy is 17600cm-

1. (1 kJ mol-1 = 349.76 cm-1.) We must compare the

total of the CFSE and the pairing energy for the two

possible configurations.


high spin (more stable)CFSE = 4 x -2/5 x 10400 + 2 x 3/5 x 10400 = -4160cm-1

(-11.89 kJ mol-1)

Pairing energy (1 pair) = 1 x 17600 = 17600 cm-1 (50.32 kJ mol-1

Total = +13440 cm-1 (38.43 kJ mol-1)

low spinCFSE = 6 x -2/5 x 10400= -24960 cm-1 (-71.36 kJ mol-

1)

Pairing energy (3 pairs) = 3 x 17600 = 52800 (151.0 kJ mol-1)

Total = +27840 cm-1 (79.60 kJ mole-1)


Tetrahedral complexes

• Splitting order or reversed. eg is now lower energy

and t2g is hgher energy

• Because a tetrahedral complex has fewer ligands,

the magnitude of the splitting is smaller. The

difference between the energies of the t2g and eg

orbitals in a tetrahedral complex (t) is slightly less

than half as large as the splitting in analogous

octahedral complexes (o)

• Dt = 4/9Do


Tetrahedral Ligand Arrangement

Dt = 4/9Do

Mostly forms high spin complxes


Octahedral Crystal Field Splitting


Square-planar Complexes-D4h


Generalizations about Crystal Field Splittings• The actual value of D depends on both the metal

ion and the nature of the ligands:• The splitting increases with the metal ion

oxidation state. For example, it roughtly doubles going from II to III.

• The splitting increases by 30 - 50% per period down a group.

• Tetrahedral splitting would be 4/9 of the octahedral value if the ligands and metal ion were the same.


Spectrochemical Series for Ligands

• It is possible to arrange representative ligands in an order of increasing field strength called the spectrochemical series:

I¯ < Br¯ < -SCN¯ < Cl¯ < F¯ < OH¯ < C2O42¯ < H2O < -

NCS¯ < py < NH3 < en < bipy < o-phen < NO2¯ < CN¯ < CO


Spectrochemical Series for Metals

It is possible to arrange the metals according to a spectrochemical series as well. The approximate order is

Mn2+ < Ni2+ < Co2+ < Fe 2+ < V2+ < Fe3+ < Co3+ < Mn3+ < Mo3 + < Rh3 + < Ru3 + < Pd4+ < Ir3+ < Pt 3+


Spectrum of [Ti(H2O)6]3+.

d1

: t2g1

eg0

–> t2g0

eg1


Hydration Enthalpy.

• M2+(g) + 6 H2O(l) = [M(O2H)6]2+(aq)


Irving-Williams Series


Ligand Field Splitting and Metals

the transition metal also impacts Do increases with increasing oxidation number

Do increases as you move down a group (i.e. with increasing principal quantum number n)


MO forML6 diagram Molecules

0


Ligand Field Stabilization Energies

LFSE is a function of Do

weighted average of the splitting due to the

fact that they are split into groups of 3 (t2g)

and 2 (eg)


Weak Field vs. Strong Fieldnow that d orbitals are not degenerate how do we know what an electronic ground state for a d metal complex is? need to determine the relative energies of pairing vs. Do


Splitting vs. Pairing

when you have more than 3 but fewer than 8 d

electrons you need to think about the relative merits

pairing vs. Do

• high-spin complex – one with maximum number of unpaired electrons

• low-spin complex – one with fewer unpaired electrons


Rules of Thumb for Splitting vs Pairing

• depends on both the metal and the ligands

• high-spin complexes occur when o is small Do is small when:

• n is small (3 rather than 4 or 5)– high spin only really for 3d metals

• oxidation state is low– i.e. for oxidation state of zero or 2+

• ligands is low in spectrochemical series– eg halogens


Four Coordinate Complexes: Tetrahedral

Same approach but different set of orbitals with different ligand field

• Arrangement of tetrahedral field of point charges results in splitting of energy where dxy, dzx, dyz are repelled more by Td field of negative charges

• So the still have a split of the d orbitals into triply degenerate (t2) and double degenerate (e) pair but now e is lower energy and t2 is higher.


Tetrahedral Crystal Field Splitting


Ligand Field Splitting: Dt

describes the separation between reviouslydegenerate d orbitals

• Same idea as Do but Dt < 0.5 Do for comparable systems

• So …Almost Exclusively Weak Field


Electron configurations in octahedral fields

Weak field and strong fieled cases


Tetragonal Complexes

Start with octahedral geometry and follow the

energy as you tetragonally distort the octahedron

Tetragonal distortion: extension along z and

compression on x and y

Orbitals with xy components increase in

energy, z components decrease in energy

Results in further breakdown of degeneracy

– t2g set of orbitals into dyz, dxz and dxy

– eg set of orbitals into dz2 and dx2-y2


Tetragonal Complexes


Square Planar Complexes• extreme form of tetragonal distortion• Ligand repulsion is completely removed from• z axis

Common for 4d8

and

5d8 complexes:

Rh(I), Ir(I)

Pt(II), Pd(II)


Jahn Teller Distortion

geometric distortion may occur in systems

based on their electronic degeneracy

This is called the Jahn Teller Effect:

If the ground electronic configuration of a

nonlinear complex is orbitally degenerate, the

complex will distort to remove the degeneracy

and lower its energy.


Jahn Teller DistortionsOrbital degeneracy: for octahedral geometry

these are:

– t2g3eg

1 eg. Cr(II), Mn(III) High spin complexes

– t2g6eg

1 eg. Co(II), Ni(II)

– t2g6eg

3 eg. Cu(II)

basically, when the electron has a choice between one of the two degenerate eg orbitals, the geometry will distort to lower the energy of the orbital that is occupied.

Result is some form of tetragonal distortion


Ligand Field TheoryCrystal field theory: simple ionic model, does not

accurately describe why the orbitals are raised or lowered in energy upon covalent bonding.

• LFT uses Molecular Orbital Theory to derive the ordering of orbitals within metal complexes

• Same as previous use of MO theory, build ligand group orbitals, combine them with metal atomic orbitals of matching symmetry to form MO’s


LFT for Octahedral Complexes

Consider metal orbitals and ligand group orbitals

Under Oh symmetry, metal atomic orbitals transform as:

Degeneracy Mulliken Label Atomic Orbital

2 eg dx2-y2, dz2

3 t2g dxy, dyz, dzx

3 t1u px, py, pz

1 a1g s


Sigma Bonding: Ligand Group Orbitals


Combinationsof Metal andLigand SALC’s


Molecular Orbital Energy Level Diagram: Oh


PI Bonding

pi interactions alter the

MOELD that results from

sigma bonding

• interactions occur between

frontier metal orbitals and the

pi orbitals of L

• two types depends on the ligand

–pi acid - back bonding accepts e- density from M

–pi base -additional e- density donation to the M

• type of bonding depends on relative energy level

of pi orbitals on the ligand and the metal orbitals


: PI Bases and the MOELD Oh

pi base ligands

contribute more

electron density to

the metal

• t2g is split to form a

bonding and

antibonding pair of

orbitals

Do is decreased

• halogens are good

pi donors


PI Acids and the MOELD: Oh• pi acids accept electron

density back from the

metal

• t2g is split to form a

bonding and antibonding

pair of orbitals

• the occupied bonding

set of orbitals goes

down in energy so ..

• Do increases

• typical for phosphine

and carbonyl ligands


Magnetic Properties of Atoms • a) Diamagnetism? • Repelled by a magnetic field due to paired electrons.

b)Paramagnetism?• attracted to magnetic field due to un-paired electrons.

c) Ferromagnetism? • attracted very strongly to magnetic field due to un-paired

electrons. • d)Anti-ferromagnetic?• Complete cancelling of unpaired electrons in magnetic

domains


Magnetic Suceptibility Vs Temperature


Types of magnetism


Magnetic PropertiesA paramagnetic substance is characterised

experimentally by its (molar) magnetic susceptibility, cm. This is measured by

suspending a sample of the compound under a sensitive balance between the poles of a powerful electro-magnet,


Number of Unparied ElectronsThe magnetic moment of the substance is given by

the Curie Law:

m = 2.54(cmT)½ (in units of Bohr magnetons)

The formula used to calculate the spin-only magnetic moment can be written in two forms

m= n(n+2) B.M.


Magnetic Properties of Atoms • Paramagnetism?• Ferromagnetism?• Diamagnetism?• Gouvy Balance


Octahedral Complexes


Tetrahedral Complexes

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