The colour of Co(II) complexes.

The colour of Co(II) complexes has interested chemists for many years and the pale-pink, octahedral to bright-blue, tetrahedral colour change is seen in such devices as weather guides and in the dye in silica gel dessicant used in the laboratory.

Assignment of the bands for these spectra can present some problems however where the different stereochemistries are interpreted on each side of an F Orgel diagram.
interpretation of Co(II) spectra

For a typical tetrahedral complex, [CoCl4]2- and assuming Δt = 4/9 Δo where Δo is around 9000 cm-1 then we can predict that the transition
4T2 ← 4A2 should be observed below 4000 cm-1. Only 1 band is seen in the visible region at 15,000 cm-1 although a full scan from the IR through to the UV reveals an additional band at 5,800 cm-1. (ε value for the 15,000 band is ~60 m2 mol-1). The lower energy band must therefore correspond to
4T1(F) ← 4A2 and the other to 4T1(P) ← 4A2 (which shows splitting thought to arise from spin-orbit coupling).

For the octahedral aqua ion, a band is observed at around 8000 cm-1 and a broad band centred around 20,000 cm-1 (ε for these bands is less than 1 m2 mol-1).
The lowest energy band must correspond to:
4T2g ← 4T1g which leaves the bands at 16,000, 19,400 and 21,600 cm-1 to be assigned.
A tentative assignment puts the
4T1g(P) ← 4T1g transition at 19,400 and hence the 16,000 band is due to 4A2g ← 4T1g.
The band at 21,600 cm-1 is believed to come from spin-orbit effects.
From this Δ ~ 9000 cm-1 and B ~ 900 cm-1.

Co(II) spectra

Low spin Co(II) complexes

TS diagram for low spin Co(II) complexes
The ground term for the low spin case is 2Eg and looking at the right hand side of the TS diagram where Δ is quite large then it can be seen there are numerous doublet excited states. The quartets would now correspond to spin-forbidden states and be the weaker bands in the spectrum.
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The Department of Chemistry, University of the West Indies,
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