(i) The IUPAC name of $\left[\mathrm{Cr}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{Cl}}_2\right]\mathrm{Cl}$ is Tetraam minedichlorochromium(III) chloride.
(ii) The isomerism exhibited by this complex is the optical isomerism.
(iii) The oxidation state of $\mathrm{Ni}$ in ${\left[{\mathrm{NiCl}}_4\right]}^{2-}$ is +2 . Since chloride is a weak field ligand, it does not cause electron pairing up in opposition to Hund's maximum multiplicity rule. As a result, Ni's valence $\mathit{d}$-orbitals contain two unpaired electrons, giving the complex its paramagnetic properties. On the other hand, carbonyl is a strong field ligand and causes electron partnering up in opposition to Hund's maximisation rule. As a result, there are no unpaired electrons and the complex is hence diamagnetic.