(1) Origin of colours:
(2) d-d transition:
(3) Selection rule of d-d transition:
(4) Charge transfer:
Colour originated by charge transfer when electronic transition occurs from one part of the Complex to other part i.e such type are also called internal Redox reaction. Intensity of clour in such type transition is very high as they donot require to follow any selection rule.
(A) Charge transfer from ligands to metal:
Examples: MnO4 -1 , MnO4 -2
, CrO4 -2, Cr2O7 -2 ,
[Fe(H2O)5(NO)]+2 , Na4[Fe(CN)5(NOS)].
(B) Charge transfer from metal
to ligands:
Examples: [Fe(CO)5] , [Fe (pi-C2H5)2],
[Cr (pi-C6H6)2],
(C) Charge transfer from metal to metal:
Examples:
Prussian Blue: (Fe(III))4[Fe(II)(CN)6]3
Prussian blue is formed when ferric(Fe^3) salt reacts with ferrocyanide. Here we see both oxidation states of iron but only +2 oxidation states of iron are observed to form coordination complexes with cyanide. And the other +3 oxidation state iron ions form the net coordination compound.

Turnbull Blue: [Fe(II)]3[Fe(III)(CN)6]2
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