(a) $\mathrm{NH}_4^{+}$(Ammonium ion) can not act as ligand Ligand donate electron to central atom or they can have lone pair of electron to donate and form bond between ligand and central atom. But $\mathrm{NH}_4^{+}$do not have lone pair of electron to donate. (b) Complex $[\mathrm{Co}(\mathrm{NH}_3)_5(\mathrm{NO}_2)]\mathrm{Cl}_2$ is Red in colour when it is present in hydrated form. It absorb moisture and turn into Red colour. It is showing linkage isomerism- $\underset{\text{Pentaamine N-nitro cobalt (III) Chloride}}{[\mathrm{CO}(\mathrm{NH}_3)_5(\mathrm{NO}_2)]\mathrm{Cl}_2} \rightleftharpoons$ $\underset{\text{Penta amine o-nitro cobalt (III) Chloride}}{[\mathrm{CO}(\mathrm{NH}_3)_5(\mathrm{ONO})]\mathrm{Cl}_2}$