AIPMT 2010 Physics and Chemistry Paper

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)