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