1

Geometric Isomerism

• cis => 2 adjacent ligands• trans => 2 ligands across the center of

coordination sphere

[PtCl2(NH3)2]

Geometric Isomerism• fac (facial) => three identical ligands

occupying the corners of a common triangular surface

• mer (meridional) => three identical ligands occupying three consecutive corners of a square plane

Cl

Cl

Cl

Cl

CL

CL

fac mer

Enantiomers of Oh complexesView down 3-fold axisChelating ligands define left or right handHelix rotation

Lower case letters are used for mirror imageStructures

δ λ

δ λ

D4h has inversion:This causes loss ofchirality

d-orbitals

2

a1g Molecular Orbitals

t1g Molecular Orbitalseg Molecular Orbitals

Octahedral Field Splitting Pattern Splitting ofd-Orbitals in Oh Field

3

Reducing axial ligand repulsions splitsThe degenerate eg and t2g orbitals

Jahn-Teller distortion

Octahedral MO

Diagram

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

t12g e0

g is ground state

1300 243 oKJcmmol

− = ≈ ∆

1max 20,300 243 o

KJcmmol

λ −= = ≈ ∆

Spectral Effects of Ligands and Metals

• Different binding atoms of ligands exert different repulsions on the metal orbitals

• Different oxidation states of the metal-increases with increasing oxidation number

• There is no quantified measure of energy changes as a result of these factors from crystal field theory

o∆

Spectral characteristics of octahedral Co complexes

Beer-Lambert Law: logo

I A bcI

ε= =

Io = light entering the substanceI = attenuated light exiting the substanceA = absorbanceb = path length, cmc = molar concentration

= transition probability (extinction coefficient)ε

o∆ Determines the color of complexes: t2g – eg

The energy of the wavelength corresponding tois

o∆

E hν=

4

Variation of ∆o

Metal ions of equal oxidation stateare placed in a spectrochemicalSeries:Mn(II)<Ni(II)<Co(II) <Fe(III)<Cr(III)<Co(III)<Ru(III)<Mo(III)<Rh(III)<Pd(III)<Ir(III)<Pt(IV)

Weak vs. Strong Field Splitting

Crystal Field Splitting Diagrams

Low spin High spin

Tetrahedral ML4

Tetrahedral ligandsImpinge upon the dOrbitals to a smallerDegree than Oh geometry

5

Relationship of ∆t and ∆o

Relationship of Octahedral vs Square Planar Geometry

π-Orbitals in L-M-LOctahedral MO Diagram, with π-Bonding

Octahedral MO Diagram, with π-Bonding Pi bonding explains field strength of ligands

6

Pi interactions are important in organo-metallic reactions

Pi backbonding: metalActs as Lewis base andProvides electron densityTo empty pi* orbitals ofligand

Microstate Term SymbolsLigand Field Theory

Mulliken symbols fromIrreducible tables

Donation ofe- density

7

Atomic termSymbolLS coupling

Symmetry label hasSame multiplicity asParent term symbol.T2g and E2g derive fromD term symbol: assymetricDegenerate population

Irreducible has same symmetryAs term symbol: excited statesWith same multiplicity are More probable

F: A2g + T1g + T2gP: T1g

t2g2

t2g1eg

1

e22

LS coupling overcomeBy ligand strength

B: Racah parameter-amount of repulsion between terms of samemultiplicity

8

Forbidden transitions are very weak (of low probability)

Transition metal complexes have color as a result of vibronicdistortions away from symmetry

High and low spin complexes have unique propertiesincluding magnetic field interactions, ligand association, and Spectral differences.

(a) paramagnetism,(b) ferromagnetism, (c) antiferromagnetism,

and (d) ferrimagnetism

Orientation of d-metal complexes in solids produces bulk magneticProperties. Magentic field of complex is measured in Bohr magnetons:

12( ( 1)n nµ = +

LFSE

Pairing energyvs LFSE

9

At room temperature, the observed value of µeff for [Cr(en)3]Br2 is 4.75 µB. Is the complex high- or low-spin?

[Cr(en)3]+2 has Cr+2, a d4 case_ _

_ eg*

……………………...……………………

t2ghigh spin

low spin 4 unpaired e-

2 unpaired e-

µ = (n(n + 2))1/2

n = 2 for low spin; µ = (8)1/2 = 2.83n = 4 for high spin; µ = (24)1/2 = 4.90

Ligand-field Theory

Octahedral Complexes_ _eg *

_ _ _ _ __ _ _ _ _

_ _ _t2g energy

free ion Hypothetical ion octahedralin a spherically field

symmetric field

Ligand-field Stabilization Energy

LFSE = x(-4Dq) + y(+6Dq)where

x = number of electrons in lower levelsy = number of electrons in upper levels

Ligand-field Stabilization Energy

Octahedral complexes_ _ _ _ _ _ eg

* ___ ___

+6Dq………………………………………………………………. 0 10Dq

-4DqX _ _ X X _ X X X t2g ___ ___

d1 d2 d3

-4Dq -8Dq -12Dq LFSE

LFSE = x(-4Dq) + y(+6Dq)

Ligand-field Stabilization Energy

Octahedral complexesd4 _ _

X _ eg*

………………………………………………

X X X t2g

XX X X weak field

strong field

-16Dq -6Dq LFSE

LFSE = x(-4Dq) + y(+6Dq)

Ligand-field Stabilization Energy

Octahedral complexesd5 _ _

X X eg*

………………………………………………

X X X t2gXX XX X weak field

strong field

-20Dq 0Dq LFSE

LFSE = x(-4Dq) + y(+6Dq)

10

Ligand-field Stabilization Energy

Octahedral complexesd6 _ _

X X eg*

………………………………………………

XX X X t2g

XX XX XX weak field

strong field

-24Dq -4Dq LFSE

LFSE = x(-4Dq) + y(+6Dq)

Ligand-field Stabilization Energy

Octahedral complexesd7 X _

X X eg*

………………………………………………

XX XX X t2g

XX XX XX weak field

strong field

-18Dq -8Dq LFSE

LFSE = x(-4Dq) + y(+6Dq)

Ligand-field Stabilization Energy

Octahedral complexesX X XX X XX XX eg

* ___ ___

+6Dq………………………………………………………………. 0 10Dq

-4DqXX XX XX XX XX XX XX XX XX t2g ___ ___

d8 d9 d10

-12Dq -6Dq 0Dq LFSE

LFSE = x(-4Dq) + y(+6Dq)

Spectrochemical Series

Ligand Field StrengthCN- > phen ~ NO2

- > en > NH3 ~ py > H2O > C2O4

-2 > OH- > F- > S-2 > Cl- > Br- > I-