Concept of colour in metal complexes :

(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: Fe4[Fe(CN)₆]₃    

Turnbull Blue: Fe4[Fe(CN)₆]₃

Related Question:

(1) Although both [Mn(H2O)6]2+ and [FeF6]3- have a d5 configuration and high-spin complexes. But the dilute solutions of Mn2+ and Fe +3 complexes are therefore colorless. Why?

(2) Which of the Complex of the following pairs has the highest value of CFSE?

(3) Colour of Complexes due to charge transfer:

(4) Why violet colour of [Ti(H2O)6]Cl3 disapear (colourless) on heating heating ? 

(5) Why [Ni(CN)4]-2 is colourless while [Ni(H2O)4]-2 although both have +2 oxidation state and 3d*8 configuration ?

(6) Why [FeF6]3– is colourless whereas [CoF6]3– is coloured ? 

(7) Why Fe(CO)5 is colourless while Fe(bipy)(CO)3 is intensely purple in colour ?

(8) Why all the tetrahedral Complexes are high spin Complexes ?