CHM 501 Lecture


Spectroscopy of Transition Metal Complexes

In multielectron atoms, the transition an electron from a ground state to an excited state involves energy changes that include the difference in the orbital energies and the changes in electron-electron repulsion. This makes understanding electronic spectroscopy of transition metal complexes more complicated than just moving an electron from a t2g orbital to an eg and saying the energy required is 10Dq.

Consider the free ions: the electron occupation of the orbitals lead to different energies in most cases.

d1 _|_ ___ ___ ___ ___ = ___ _|_ ___ ___ ___ = ___ ___ _|_ ___ ___ etc – this is called a 2D state (the superscript refers to the spin quantum number = 2S + 1 = the spin degeneracy)

d2 _|_ _|_ ___ ___ ___ = ___ _|_ _|_ ___ ___ = _|_ ___ _|_ ___ ___ etc – this is the 3F state

_||_ ___ ___ ___ ___ = ___ _|| ___ ___ ___ etc – this is the 1D state

There are a total of 5 different states for d2

For free ions, the states are

d1 2D

d2, d8 3F, 3P, 1G, 1D, 1S

d3, d7 4F, 4P, 2H, 2G, 2F, 2D, 2D, 2P

d4, d6 5D, 3H, 3G, 3F, 3F, 3D, 3P, 3P + singlet states

d5 6S, 4G, 4F, 4D, 4P, + doublet states

The energies of all the states can be found using Condon-Shortley parameters (F0, F2, F4) or Racah parameters (A, B, C). Racah parameters are usually preferred because the difference in energies of the lowest energy states of the same spin degeneracy can written in terms of just B.

In an Oh field these state split into more states, which can be found using group theory.

D becomes T2g + Eg

F becomes T1g + T2g + A2g

P becomes T1u

S becomes A1g

The states in an Oh field also retain the spin degeneracy. For example, in a d2 case the free ion state 3F becomes 3T1g + 3T2g + 3A2g in an Oh field.

Transitions between are allowed between states of the same spin degeneracy plus the selection rules based on group theory: the direct product of the ground state, the excited state, and the electric field operator (the irreducible rep(s) for x, y, or z) must contain the totally symmetric representation (a1g in Oh). For true Oh complexes this never happens so all electronic spectra in Oh complexes are relatively weak.

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

Exact formulas for some cases have been published: A. B. P. Lever, J. Chem. Ed., 1968, 45, 711 – 712; Y.-S. Dou, J. Chem. Ed., 1990, 67, 134.

For d3 and d8 complexes three spin allowed transitions are predicted at energies given by

ν1 = 10Dq

ν2 = 7.5B + 15Dq – 0.5(225B2 + 100Dq2 -180BDq)½

ν3 = 7.5B + 15Dq + 0.5(225B2 + 100Dq2 -180BDq)½

If all three transitions are observed, then B can easily found from 15B = ν1 + ν2 – 3ν3

If ν3 is obscured by a CT band, which is often the case, then B can be found by 3B = [(ν2 – 2ν1)(ν2 – ν1)]/[5ν2 – 9ν1]

Likewise, for d2 and d7(hs) complexes three spin allowed transitions are predicted at energies given by

ν1 = –7.5B + 5Dq + 0.5(225B2 + 100Dq2 -180BDq)½

ν2 = –7.5B + 15Dq + 0.5(225B2 + 100Dq2 -180BDq)½

ν3 = (225B2 + 100Dq2 -180BDq)½

If all three transitions are observed, then 10Dq = ν2 – ν1 and 15B = ν3 + ν2 – 3ν1

If ν3 is obscured by a CT band, then B can be found by 3B = [(ν2 – 2ν11]/[9ν1 – 4ν2]

Tanabe-Sugano diagrams

When simple formulas don't work or there are spin-forbidden transitions observed, then Tanabe-Sugano diagrams are used