Isomerism & Crystal Field Isomerism & Crystal Field TheoryTheory

Chapter 24 – Lecture 3Chapter 24 – Lecture 3

Chemistry 123Spring 2008

Dr. Woodward

IsomersIsomers

Different ligands

make bonds to the metal

Ligands bond to the metal

through different atoms

Ligands have

different (ligand)

neighbors

Molecules are non-

superimposable mirror images

Linkage IsomersLinkage Isomers

Geometric IsomersGeometric Isomers

Optical IsomersOptical Isomers

Intra-atomic (d to d) excitationsIntra-atomic (d to d) excitations

In these complexes the color comes from absorption of light that leads to excitation of an electron from one d-orbital to a different d-orbital on the same metal cation.

CuSO4NiSO4CoSO4FeSO4MnSO4 ZnSO4

[M(H[M(H22O)O)66]]n+n+ Complex ions Complex ions

MMHH22OO

MOMO66 Octahedron Octahedron

ddzz22 & d & dxx22-y-y22 orbitals (e orbitals (egg))• Point directly at the ligands • Stronger (repulsive) interaction with the ligands

Crystal Field Splitting Crystal Field Splitting (Octahedron)(Octahedron)

ddzz22 ddxx22-y-y22

ddxyxy ddyzyz ddxzxz

Electrons in d-orbitals are repelled from the electrons in the ligands. The repulsion is stronger for electrons

in d-orbitals pointing directly at the ligands.

En

erg

y

ddxyxy, d, dyzyz & d & dxzxz orbitals orbitals (t(t2g2g))• Point between the ligands • Weaker (repulsive) interaction with the ligands

Energy Levels of d-orbitals in an Energy Levels of d-orbitals in an octahedronoctahedron

ddxyxyddyzyz ddxzxz ddz2z2 ddx2-y2x2-y2

ddxyxy ddyzyz ddxzxz ddz2z2 ddx2-y2x2-y2

Free Cr3+ ion

Cr3+ ion with a spherical crystal

field

Cr3+ ion in an octahedral crystal field

En

erg

y

ddxyxy ddyzyz ddxzxz

ddz2z2 ddx2-y2x2-y2

Electronic ExcitationElectronic Excitation

ddxyxy ddyzyz ddxzxz

ddzz22ddxx22-y-y22

t2g

eg

= Crystal Field Splitting Energy

En

erg

y

ddxyxy ddyzyz ddxzxz

light (photon)

ddzz22ddxx22-y-y22

0.0

0.1

0.2

0.3

0.4

0.5

0.6

250 300 350 400 450 500 550 600 650 700 750

Wavelength (nm)

Abs

orba

nce

[Cr[Cr3+3+(H(H22O)O)66]]3+3+ Absorption Spectra Absorption Spectra

Nonbonding Metal d MO’s

Antibonding (*) Metal-Ligand MO’s

oct