IIT JEE Advanced 2008 Paper 2

The colourless crystals of anhydrous CuF2 turn blue when exposed to moist air. In coordination complexes, colour arises due to d-d transitions. In the complexes given in options (B) and (D), the oxidation state of Cu is (I) and the electronic configuration of Cu+ is [Ar] 3d10 or completely filled 3d subshell. So on crystal field splitting in presence of ligands, there is no electron that can be promoted to higher level, hence d-d transitions are not possible and complexes are colourless.