Explain why hexacayano complexe of metals in their +2 oxidation state are usually yellow , but the corresponding hexa aqua compounds are often blue or green .

Explanation

In hexacyano complexes, cyanide is a strong ligand so causes greater crystal field splitting to a greater extant hence d-d transition requires smaller wavelength and the complimentary colour is yellow. While HLO is a weak ligand in case of hexaaqua complex and causes smaller. Crystal field splitting hence d-d transition requires large wavelength and the complementary colour is blue or green