Among the following which are ambidentate ligands?
(i) ${\mathrm{NO}}_2^{-}$ (ii) ${\mathrm{NO}}_3^{-}$ (iii) ${\mathrm{C}}_2{\mathrm{O}}_4^{2-}$ (iv) ${\mathrm{SCN}}^{-}$

Ligands which can have more than one donor site but only one site used at one time are called ambidentate ligands. ${\mathrm{NO}}_2^{-}$ and ${\mathrm{SCN}}^{-}$ are such ligands.