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^– < S^2– < SCN^– < Cl^– < NO₃^– < F ^– < ox^2– < 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)]^½μ_B

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

Since each unpaired electron has S = ½

μ = [n(n+2)] ^½ μ_B

n = number of unpaired spins

Measurement of 10Dq

Usually done spectroscopically, move electron from t_2g to e_g 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 M–L+	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