(a) Linkage isomerism is shown by the complex $[\mathrm{Co}(\mathrm{NH}_3)_5(\mathrm{SCN})]^{2+}$(b) $[\mathrm{NiCl}_4]^{2-}$ contains $\mathrm{Ni}^{2+}$ ion having $3 d^{8}$ configuration.Ground state $\mathrm{Cl}^{-}$ is a weak field ligand. Hence outer $4 s$ -and $4 p$ -orbitals are used in hybridization.It has two unpaired electrons hence it is paramagnetic whereas $[\mathrm{Ni}(\mathrm{CO})_4]$ contains $\mathrm{Ni}(0)-3 d^{8} 4 s^{2}$ configuration$\mathrm{CO}$ is a strong field ligand hence $4 s$-electrons will shift to $3 d$-orbital making $4 s$-orbital vacant. The complex has all paired electrons hence it is diamagnetic.(c) In tetrahedral complexes, $s p^{3}$ hybridization occurs. The $3 d$ orbitals are untouched, so unpaired electrons are available always which give high spins. Therefore, low spin tetrahedral complexes are formed rarely.