Similarly, to carbonyl ligands, isocyanides (C≡N-R) are good ligands for metals in low oxidation states. Due to the weaker π -accepting properties of isocyanides in comparison with carbonyl (CO) ligands, there exist only very few examples of homoleptic metal-isocyanide complexes with metals in negative oxidation states [M(CNR)6] -(for M=V, Nb, Ta), [Fe(CNR)4] 2-2 and [Co(CNR)4] -. In comparison with cyanide ligands, the σ-donating properties of alkyl isocyanide ligands are less pronounced, which is a limiting factor for the stability of metal alkyl isocyanide complexes in high oxidation states.