Transition metal nitrile complexes

Transition metal nitrile complexes are coordination compounds containing nitrile ligands. Because nitriles are weakly basic, the nitrile ligands in these complexes are often labile.

According to the Covalent bond classification method, nitriles are classified as L ligands, i.e., charge-neutral Lewis bases. With respect to HSAB theory, they are classified as soft.

Typical nitrile ligands are acetonitrile, propionitrile, and benzonitrile. The structures of [Ru(NH₃)₅(NCPh)]ⁿ⁺ have been determined for the 2+ and 3+ oxidation states. Upon oxidation the Ru-NH₃ distances contract and the Ru-NCPh distances elongate, consistent with amines serving as pure-sigma donor ligands and nitriles functioning as pi-acceptors.

Acetonitrile, propionitrile and benzonitrile are also popular solvents. Because nitrile solvents have high dielectric constants, cationic complexes containing a nitrile ligand are often soluble in a solution of that nitrile.

Some complexes can be prepared by dissolving an anhydrous metal salt in the nitrile. In other cases, a suspension of the metal is oxidized with a solution of NOBF₄ in the nitrile:

Heteroleptic complexes of molybdenum and tungsten can by synthesized from their respective hexacarbonyl complexes.

For the synthesis of some acetonitrile complexes, the nitrile serves as a reductant. This method is illustrated by the conversion of molybdenum pentachloride to the molybdenum(IV) complex:

Transition metal nitrile complexes are usually employed because the nitrile ligand is labile and relatively chemically inert. Cationic nitrile complexes are however susceptible to nucleophilic attack at carbon. Consequently some nitrile complexes catalyze the hydrolysis of nitriles to give the amides.