[FeF6]3- is a Fe (III) complex
hence [Ar] 3d5. F-
is a weak field ligand and the complex is high spin:
t2g (↑) (↑)(↑) →Δoct→ (↑)(↑)eg.
And there is 5
unpaired electrons hence it's spin multiplicity = (2S+1) = 6 , and multiplicity of a excited is cannot be six thus The transitions in Fe +3 ion are spin
forbidden and are extremely weak so as to make [FeF6]3-
almost colorless.
[CoF6] 3- this is a Co (III)
complex [Ar] 3d6. Again F-
is a weak field ligand and [CoF6]3- is one of only two
common cmplxs that is high spin:
t2g (↑↓) (↑)(↑) →Δoct→ (↑)(↑) eg
Only one band is
expected namely the t2g→eg transition (d1). The complex
is blue consistent with this
transition being in the low energy red.
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