For the absorption of visible light, presence of unpaired d-electrons is the necessity. (1) [Ni(CN)4]2−,Ni is present as Ni2+. Ni2+=[Ar]3d84s0
(Pairing occurs because CN−is a strong field ligand). Since, in [Ni(CN)4]2−, no unpaired electron is present in d-orbitals, it does not absorb visible lights. (2) ln[Cr(NH3)6]3+,Cr is present as Cr3+. Cr3+=[Ar]3d34s0 (Three unpaired electrons) (3) ln[Fe(H2O)6]2+,Fe is present as Fe3+. Fe2+=[Ar]3d64s0 (Four unpaired electrons) (4) In [Ni(H2O)6]2+,Ni is present as Ni2+. Ni2+=[Ar]3d34s0 (Two unpaired electrons)