(a) In the complex [Fe(CN)6 ]3– the iron is in +3 oxidation state having d5 configuration, so even after pairing due to strong field cyanide ligands one electron will remain unpaired and hence it is weakly paramagnetic. While in the complex [Fe(CN)6 ]4– iron is in +2 oxidation state and having d6 configuration , so after pairing due to strong field cyanide ligand no unpaired electron ***, so is diamagnetic.
(b) Octahedral complexes requires d2sp3 hybridisation for being inner d–orbital complexes, which will not be available in d8 configuration of Ni2+, hence all complexes are outer d–orbital complexes.
(c) Co2+ has d7 configuration and only strong field ligands can excite the last electron of the d orbital to higher orbitals and can form more stable inner d–orbital complexes with d2sp3 hybridisation.
(d) In [Ni(CO)4 ], Ni is in 0 oxidation state so has 3d84s2 configuration which after pairing due to strong field CO ligands will become 3d10, and hence no d orbitals are empty, it can only undergo sp3 hybridisation leading to tetrahedral geometry. While in other given complexes Ni is in 2+ oxidation state so will form dsp2 complexes.
