Number of ambidentate ligands among the following is _________.

$$\mathrm{NO}_2^{-}, \mathrm{SCN}^{-}, \mathrm{C}_2 \mathrm{O}_4^{2-}, \mathrm{NH}_3, \mathrm{CN}^{-}, \mathrm{SO}_4^{2-}, \mathrm{H}_2 \mathrm{O} \text {. }$$

<p>Ambidentate ligands are ligands that can attach to a central metal atom through two different atoms. Let’s analyze the given ligands one by one:</p> <ul> <li><b>$\mathrm{NO}_2^{-}$:</b> This ligand can bind through either the nitrogen (N) or the oxygen (O). Therefore, $\mathrm{NO}_2^{-}$ is an ambidentate ligand.</li> <li><b>$\mathrm{SCN}^{-}$:</b> This ligand can bind through either the sulfur (S) or the nitrogen (N). Therefore, $\mathrm{SCN}^{-}$ is an ambidentate ligand.</li> <li><b>$\mathrm{C}_2\mathrm{O}_4^{2-}$ (oxalate):</b> This is a bidentate ligand, but it coordinates through both oxygen atoms only. Therefore, it is not an ambidentate ligand.</li> <li><b>$\mathrm{NH}_3$:</b> This ligand can only bind through nitrogen (N). Therefore, it is not an ambidentate ligand.</li> <li><b>$\mathrm{CN}^{-}$:</b> This ligand can bind through either carbon (C) or nitrogen (N). Therefore, $\mathrm{CN}^{-}$ is an ambidentate ligand.</li> <li><b>$\mathrm{SO}_4^{2-}$ (sulfate):</b> This ligand typically coordinates through oxygen atoms. It is not considered an ambidentate ligand.</li> <li><b>$\mathrm{H}_2\mathrm{O}$:</b> This ligand can only bind through oxygen (O). Therefore, it is not an ambidentate ligand.</li> </ul> <p>Based on this analysis, the ambidentate ligands are $\mathrm{NO}_2^{-}$, $\mathrm{SCN}^{-}$, and $\mathrm{CN}^{-}$.</p> <p>Therefore, the number of ambidentate ligands among the given options is <b>3</b>.</p>