Why do transition metals form coloured complexes ?

Explanation

Most of the compound of transition elements are coloured due to the presence of unpaired electrons in d-subshell of transition metal ions. It can be explained with the help of crystal filed theory. For example, in an octahedral complex `[Ti(H_(2)O)_(6)]^(3+)`, under the influence of ligands (`H_(2)O` molecules ),all the five degenerate d-orbitals of penultimate shell of transition metal ion `Ti^(3+)` split up in to two degenerate sets, one containing `d_(x^(2)-y^(2))` and `d_(z)^(2)` (with higher energy ) and the other containing `d_(xy),d_(yz),d_(zx)` ( with lower energy ). When white light falls, a part of it corresponding to a certain wavelenght ( yellow in this case ) is abosrbed. Due to small energy difference, electronic excitations take place from one set to another (d-d transition ). The remaining colours of white light ( red and blue ) are transmitted and the compound appears purple coloured.