(a) Which of the following species cannot act as a ligand? Give reason.

$$\mathrm{OH}^{-}, \mathrm{NH}_4^{+}, \mathrm{CH}_3 \mathrm{NH}_2, \mathrm{H}_2 \mathrm{O}$$

(b) The complex $$\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5\left(\mathrm{NO}_2\right)\right] \mathrm{Cl}_2$$ is red in colour. Give IUPAC name of its linkage isomer.