(a) Linkage isomerism is shown by the complex [Co(NH3)5(SCN)]2+
(b) [NiCl4]2− contains Ni2+ ion having 3d8 configuration.
Ground state
Cl− is a weak field ligand. Hence outer 4s -and 4p -orbitals are used in hybridization.It has two unpaired electrons hence it is paramagnetic whereas [Ni(CO)4] contains Ni(0)−3d84s2 configurationCO is a strong field ligand hence 4 s-electrons will shift to 3d-orbital making 4 s-orbital vacant.
The complex has all paired electrons hence it is diamagnetic.
(c) In tetrahedral complexes, sp3 hybridization occurs. The 3d orbitals are untouched, so unpaired electrons are available always which give high spins. Therefore, low spin tetrahedral complexes are formed rarely.