(i) Complex $[\mathrm{Co}(\mathrm{NH}_3)_6]\;\;[\mathrm{Cr}(\mathrm{CN})_6]$ 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 $[\mathrm{Ni}(\mathrm{H}_2\mathrm{O})_6]^{2+},\mathrm{H}_2\mathrm{O}$ is a weak field ligand. So there are unpaired electrons in $\mathrm{Ni}^{2+}$. In this complex, the $d$ electrons from the lower energy level can be excited to the higher energy level. The possibility of $d$ - $d$ electrons is present. Therefore, $\mathrm{Ni}(\mathrm{H}_2\mathrm{O}_6)]^{2+}$ is coloured. In $[\mathrm{Ni}(\mathrm{CN})_4]^{2-}$, the electrons are all paired as $\mathrm{CN}^{-}$is a strong ligand. So, $d-d$ transition is not possible in $[\mathrm{Ni}(\mathrm{CN})_4]^{2-}$. Hence, it is colourless. (iii) The IUPAC name of $[\mathrm{Co}(\mathrm{NH}_3)_5(\mathrm{CO}_3)]\mathrm{Cl}$ is Pentaamminecarbonatocobalt (III) chloride.