Statement I: -NO₂ and -CN are ambidentate ligands

Statement II: An ambidentate ligand has two sites on different species available for coordination but only one is used at a time

Both Statement I and statement II are correct and statement II is the correct explanation of statement I

The correct answer is option 1. Both Statement I and statement II are correct and statement II is the correct explanation of statement I.

Statement I: $-NO_2$ and $-CN$ are ambidentate ligands.

Statement I is true. $-NO_2$ (nitro) and $-CN$ (cyanide) ligands are ambidentate ligands because they can coordinate through two different atoms to the metal center. In the case of $-NO_2$, it can coordinate through either the nitrogen atom or the oxygen atom. Similarly, $-CN$ can coordinate through either the carbon atom or the nitrogen atom.

Statement II: An ambidentate ligand has two sites on different species available for coordination but only one is used at a time.

Statement II is also true. An ambidentate ligand has multiple potential coordination sites on different atoms but can coordinate with the metal center using only one site at a time. In the case of $-NO_2$, it can coordinate through either the nitrogen atom or the oxygen atom, but only one of these sites will be used to form the coordination bond with the metal ion at a given time. The same applies to $-CN$, which can coordinate through either the carbon atom or the nitrogen atom, but only one site will be used for coordination.

Statement II provides the correct explanation for why $-NO_2$ and $-CN$ are ambidentate ligands. They have multiple coordination sites available, but only one site is utilized to coordinate with the metal center at any given moment.

Therefore, the correct answer is Option 1: Both Statement I and Statement II are correct, and Statement II is the correct explanation of Statement I.