Central Tenants of Crystal Field Theory

• The metals (Lewis acids) have d orbitals that are partially filled with electrons.

• Ligands, that are Lewis bases with lone pairs, come in and form a covalent bond.

• Electrostatic repulsion between the ligand lone pairs with the

d-orbital subshell leads to higher energies

• Each metal orbital will either be stabilized or destabilizeddepending on the amount of orbital overlap with the ligand.

CFT (Octahedron)

(dz2, dx2-y2)

Orbitals point

directly at ligands

stronger repulsion

higher energy

(dxy, dyz, dxz)

Orbitals point

between ligands

weaker repulsion

lower energy

dz2 dx2y2

eg

dxy dyz dxz

t2g

E

Energy Levels of d-Orbitals in an Octahedron

• crystal field splitting energy =

• The size of is determined by;

metal (oxidation state; row of metal, for 5d > 4d >> 3d)

ligand (spectrochemical series)

Spectrochemical Series

t2g

eg

t2g

eg

I < Br < Cl < F < OH < H2O < NH3 < en < NO2 < CN < CO

[Cr(H2O)6]3+

Greater

Small Δ

Weak Field

Weak M-L interactions

Large Δ

Strong Field

Strong M-L interactions

How Do the Electrons Go In?

• Hund’s rule: one electron each in lowest energy orbitals first

• what happens from d4 to d7?

d6

Crystal Field

SplittingSpin Pairing

Δ Pvs.

High Spin vs. Low Spin

High Spin Low Spin

Co3+ (d6)

PΔ < PΔ >

High Spin vs. Low Spin Configurations

high spin low spin

d4

d5

d6

d7

High Spin vs. Low Spin Configurations

strong field

ligand

weak field

ligand

4d & 5d

always

3d metal 3d metal

linear (CN = 2)

tetrahedral (CN = 4)

square planar (CN = 4)

What About Other Geometries?

x

y

z

x

y

z

x

y

z

how do the electrons on the d orbitals

interact with the ligands in these cases?

Tetrahedral vs. Octahedral

Spherical

crystal field

Octahedral

crystal field

Energ

y Δoct

Δtet

dxy dyz dxz

dz2 dx2-y2

Tetrahedral

crystal field

Δtetrahedron ≈ (4/9) Δoctahedron

Consequently, tetrahedral complexes are always high spin.

dz2 dx2-y2

dxy dyz dxz

dxy dyz dxz dz2 dx2-y2

Square Planar d-level Splitting

xy

z

x

y

z

octahedral

(CN = 6)

square planar

(CN = 4)

All z-levels lower in

energy due to less

e− repulsion

dz2 more stable so

lower in energy than

dxy orbital

d8

Common for

4d & 5d metals

Low Spin

High Spin or Low Spin?

1) What is the

Coordination Geometry ?

Tetrahedral

(CN = 4)

High Spin

Square Planar

(CN = 4)

Low SpinOctahedral

(CN = 6)

2) Is it a 1st (3d), 2nd (4d) or

3rd (5d) row Transition Metal ?

If 1st Row

Depends on Ligand & Metal

F, Cl, Br, I = High Spin

CN, CO = Low Spin

2nd or 3rd Row

Low Spin

Orbital Splitting vs. Geometry

Electron Configurations of Complexes

[Rh(CN)2(en)2]+

[MnCl6]4

coord. # geom. d-e− config.

Electron Configurations of Complexes

[NiCl4]2

[Pt(NH3)4]2+

coord. # geom. d-e− config.

[FeCl4]2– is tetrahedral. How many unpaired

electrons will [FeCl4]2– have?

Magnetism

• paramagnetism– compounds with one or more

unpaired electrons are attracted by a magnetic field

– The more unpaired electronsthe greater the attractive force

• diamagnetism– compounds with no

unpaired electrons repel a magnetic field

– much weaker effect

Octahedral Crystal Field

d6 High Spin (i.e. Co3+)

t2g

eg

d6 Low Spin (i.e. Co3+)

t2g

eg

Color of Transition Metal Complexes

The color of these compounds comes from the absorption of light that causes an excitation of an electron from one

d-orbital to a different d-orbital on the same metal cation.

CuSO4NiSO4CoSO4FeSO4MnSO4 ZnSO4

• Spectrochemical series

– relative strength of metal ion/ligand interactions

– independent of metal

octahedral cobalt(III) complex ions

a. CN– b. NO2– c. phen d. en e. NH3 f. gly g. H2O h. ox i. CO3

2–

1,10-phenanthroline glycine

Color of Transition Metal Complexes

Absorption Of Light

• compounds absorb light if the light has the correct energy to cause an electron to move from a lower energy state to a higher energy state

octahedral d3 (i.e. Cr3+)

t2g

eg

E

oct

Light (hv)

(photon)

t2g

eg

oct

The size of oct dictates the wavelength of light needed,

and therefore, the color of the compound.

“GROUND STATE” “EXCITED STATE”

Absorption Dependence on d-electrons

Different numbers of transitions will occur for

different d-electron configurations

t2g

eg

E

d10 = colorless

t2g

eg

d0 = colorless

d1-d9 = various visible transitions