Which one of the following is an ambidentate ligand?

Solution & Explanation

\textbf{Answer:} (C) Thiocyanate. \textbf{Solution:} An ambidentate ligand has two different donor atoms but coordinates through only one of them at a time, so it can bind to the metal through either site. The thiocyanate ion, \ce{SCN-}, can coordinate either through sulfur (\ce{M-SCN}, thiocyanato-S) or through nitrogen (\ce{M-NCS}, thiocyanato-N / isothiocyanato). Having two distinct possible donor atoms makes it ambidentate (and gives rise to linkage isomerism). The distractors differ in denticity, not ambidentacy: ethane-1,2-diamine (en) and oxalate \ce{C2O4^2-} are bidentate, and \ce{EDTA^4-} is hexadentate — each binds through a fixed set of donor atoms simultaneously. Hence the ambidentate ligand is thiocyanate, option (C).