(i) Complex $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]\left[\mathrm{Cr}{\left(\mathrm{CN}\right)}_6\right]$ shows coordination isomerism. This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in the complex.
(ii) In ${\left[\mathrm{Ni}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+},{\mathrm{H}}_2\mathrm{O}$ is a weak field ligand. So there are unpaired electrons in ${\mathrm{Ni}}^{2+}$. In this complex, the $\mathit{d}$ electrons from the lower energy level can be excited to the higher energy level. The possibility of $\mathit{d}$ - $\mathit{d}$ electrons is present. Therefore, $\mathrm{Ni}\left({\mathrm{H}}_2{\mathrm{O}}_6\right){]}^{2+}$ is coloured. In ${\left[\mathrm{Ni}{\left(\mathrm{CN}\right)}_4\right]}^{2-}$, the electrons are all paired as ${\mathrm{CN}}^{-}$is a strong ligand. So, $\mathit{d}-\mathit{d}$ transition is not possible in ${\left[\mathrm{Ni}{\left(\mathrm{CN}\right)}_4\right]}^{2-}$. Hence, it is colourless.
(iii) The IUPAC name of $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5\left({\mathrm{CO}}_3\right)\right]\mathrm{Cl}$ is Pentaamminecarbonatocobalt (III) chloride.