Transition metal isocyanide complexes


Transition metal isocyanide complexes are coordination compounds containing isocyanide ligands. Because isocyanide are relatively basic, but also good pi-acceptors, a wide range of complexes are known. Some isocyanide complexes are used in medical imaging.

Scope of isocyanides

Isocyanides are electronically similar to CO but for most R groups, isocyanides are superior Lewis bases and weaker pi-acceptors. Typical isonitrile ligands are methyl isocyanide, tert-butyl isocyanidephenyl isocyanide, and cyclohexylisocyanide. 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 +. Very bulky isocyanide ligands are also known, e.g. C6H3-2,6-Ar2-NC.
Di- and triisocyanide ligands are well developed, e.g. n2. Usually steric factors force these ligands to bind to two separate metals, i.e., they are binucleating ligands. Chelating diisocyanide ligands require elaborate backbones.

Synthesis and reactions

Because of their low steric profile and high basicity, isocyanide ligands often install easily, e.g. by treating metal halides with the isocyanide. Many metal cyanides can be N-alkylated to give isocyanide complexes.
Cationic complexes are susceptible to nucleophilic attack at carbon. In this way, the first metal carbene complexes where prepared. Because isocyanides are both acceptors and donors, they stabiilize 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, n for n = -1, 0, +1.
Because isocyanides are more basic donors ligands than CO, their complexes are susceptible to oxidation and protonation. Thus, Fe5 is easily protonated, whereas its counterpart Fe5 is not.
Isocyanide has been shown to insert into metal-alkyl bonds to form iminoacyls.