Mononuclear alkyl N-heterocyclic carbene complexes of the type Fe(NHC)R2, NHC = SIPri, IPri, R = CH2SiMe3, CH2C6H5, were prepared by the alkylation of “FeCl2(THF)1.5” with one equivalent per Fe of MgR2, in the presence of the NHC. Alkylation with 0.5 equiv of MgR2 under carefully controlled conditions gave Fe(NHC)(CH2SiMe3)Cl, NHC = SIPri, IPri. Magnetic and computational studies and structural determination by X-ray diffraction reveal three-coordinate high-spin (S = 2) alkyl complexes.

A range of new N-heterocyclic carbene complexes of iron(II) and cobalt(II) have been conveniently obtained by the aminolysis of bis[bis(trimethylsilyl)amido]iron and -cobalt precursors with imidazol(in)ium salts. Whereas sterically less hindered salts produced the tetrahedral complexes [M(carbene)2Cl2] (M = Fe), bulkier salts gave the three-coordinate [M(carbene){N(SiMe3)2}Cl] (M = Fe, Co), which serve as versatile precursors to a range of derivatives; mechanistic aspects of the reaction are discussed.