Transition metal isocyanide complexes

file:Tc CNCH2CMe2 6Cation.png|thumb|Technetium (^99mTc) sestamibi is used in nuclear medicine imaging.
Transition metal isocyanide complexes are coordination compounds containing isocyanide ligands. Several thousand isocyanides are known, but the coordination chemistry is dominated by a few ligands. Common isonitrile ligands are methyl isocyanide, tert-butyl isocyanide, phenyl isocyanide, and cyclohexylisocyanide.
Some isocyanide complexes are used in medical imaging.

Ligand properties

According to the Covalent bond classification method, isocyanides are classified as L ligands, i.e., charge-neutral Lewis bases. With respect to HSAB theory, it is classified as soft.
Compared to CO, most isocyanides are superior Lewis bases and weaker pi-acceptors. Trifluoromethylisocyanide is the exception, its coordination properties are very similarly to those of CO. Isocyanide complexes often mirror the stoichiometry and structures of metal carbonyls. Like CO, isocyanides engage in pi-backbonding. The M-C-N angle provides some measure of the degree of backbonding. In electron-rich complexes, this angle is usually deviates from 180°. Unlike CO, cationic and dicationic complexes are common. RNC ligands are typically terminal, but bridging RNC ligands are common. Bridging isocyanides are always bent. General trends can be appreciated by inspection of the homoleptic complexes of the first row transition metals.
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

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

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.

Reactions

Typically, isocyanides are spectator ligands, but their reduced and oxidized complexes can prove reactive by virtue of the unsaturated nature of the ligand
Cationic isocyanide complexes are susceptible to nucleophilic attack at carbon. In this way, the first metal carbene complexes where prepared.

Protonation

Because isocyanides are more basic donors ligands than CO, their complexes are susceptible to oxidation and protonation. Thus, is easily protonated, whereas its counterpart is not:
Some electron-rich isocyanide complexes protonate at N to give aminocarbyne complexes:
Isocyanides sometimes insert into metal-alkyl bonds to form iminoacyls.

Redox

Because isocyanides are both acceptors and donors, they exhibit more reversible redox than metal carbonyls. This aspect is illustrated by the isolation of the homoleptic vanadium hexaisocyanide complex in three oxidation states, i.e., ⁿ for n = -1, 0, +1.

IR spectroscopy

The ν_C≡N band in isocyanides is intense in the range of 2165–2110 cm⁻¹. The value of ν_C≡N is diagnostic of the electronic character of the complex. In complexes where RNC is primarily a sigma donor ligand, ν_C≡N shifts to higher energies vs free isocyanide. Thus, for, ν_C≡N = 2152, 2120 cm^−l. In contrast, for the electron-rich species Fe₂₉, ν_C≡N = 2060, 1920 cm^−l for the terminal isocyanide ligands.