Transition metal isocyanide complexes

Trifluoromethylisocyanide is the exception, its coordination properties are very similarly to those of CO. Because the CNC linkage is linear, the cone angle of these ligands is small, so it is easy to prepare polyisocyanide complexes.

Many complexes of isocyanides show high coordination numbers, e.g. the eight-coordinate cation [Nb(CNBu−t)6I2]+.

Because isocyanides are both acceptors and donors, they stabilize a broader range of oxidation states than does CO.

This advantage is illustrated by the isolation of the homoleptic vanadium hexaisocyanide complex in three oxidation states, i.e., [V(CNC6H3-2,6-Me2)6]n for n = -1, 0, +1.

Thus, Fe(tBuNC)5 is easily protonated, whereas its counterpart Fe(CO)5 is not:[8] Some electron-rich isocyanide complexes protonate at N to give aminocarbyne complexes:[12] Isocyanides sometimes insert into metal-alkyl bonds to form iminoacyls.

General trends can be appreciated by inspection of the homoleptic complexes of the first row transition metals.

In complexes where RNC is primarily a sigma donor ligand, νC≡N shifts to higher energies vs free isocyanide.