CHM 401

 

Influences on Dq:

metal : charge, size (Z*)

ligands : charge, orbitals available for bonding (σ, π), ring formation

Spectrochemical series

ligands ordered by relative size of Dq for any metal ion ligands ordered by ligand field strength

I < Br < S2– < SCN < Cl < NO3 < F < ox2– < H2O < SCN < CH3CN < NH3 < en < bipy < phen < NO2 < PPh3 < CN < CO


High spin vs low spin

Complexes are high spin if Dq is small : weak field case

Complexes are low spin if Dq is large : strong field case

Magnetic susceptibility measurements, which measure the size of the magnetic moment, are used to distinguish the spin state.

The magnetic moment reflects the total available angular momentum in transition metal complexes (especially first row), most of this comes from spin (the orbital contribution is said to be quenched):

μ = g[S(S+1)]½μB

g ~ 2 (fundamental constant); μB = Bohr magneton

Since each unpaired electron has S = ½

μ = [n(n+2)] ½ μB

n = number of unpaired spins

Measurement of 10Dq

Usually done spectroscopically, move electron from t2g to eg orbital with no spin change

hν = 10Dq in this case

Because of electron–electron repulsion, the lowest energy transition is not always equal to 10Dq.

Configuration

Lowest Energy Spin–Allowed Transition

d1

10Dq

d2

8Dq

d3

10Dq

d4(hs)

10Dq

d4 (ls)

~ 9Dq

d5 (hs)

No spin–allowed transition

d5 (ls)

~ 8.5 Dq

d6(hs)

10Dq

d6 (ls)

~ 9Dq

d7 (hs)

10Dq

d7 (ls)

~ 9Dq

d8

8Dq

d9

10Dq

Complication : charge transfer transitions

M–L M+L

Metal to Ligand Charge Transfer (MLCT)

M–L ML+

Ligand to Metal Charge Transfer (LMCT)

CT transitions are usually much more intense than d–d transitions so can be distinguished by molar absorptivity

ε (CT) ~ 103 – 104 L/mol–cm

ε (d–d spin allowed) ~ 101 – 102 L/mol–cm

ε (d–d spin forbidden) ~ 10–1 – 100 L/mol–cm