Half-sandwich iron complexes of N-heterocyclic carbenes, Cp*Fe(L )Cl (2a; L = 1,3-dimesitylimidazol-2-ylidene, 2b; L = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Cp* = η -C Me ), have been synthesized by the reaction of Cp*Fe{N(SiMe ) } (1) with the corresponding imidazolium salts. Treatment of 2a with either methyllithium or phenyllithium replaces the chloride with either a methyl or a phenyl group, generating Cp*Fe(L )R (3a; R = Me, 3b; R = Ph). These complexes, in turn, undergo cyclometalation at elevated temperatures, and Cp*Fe{κ -(C,C)-L′ } (4; L′ = CH C H -3,5-Me -2-(3-mesityl-imidazol-2-ylidene-1-yl)) was isolated. On the other hand, methylation of 2b at room temperature leads directly to the formation of a cyclometalated complex, Cp*Fe{κ -(C,C)- L′ } (6; L′ = CH CH(CH )(3-isopropyl-4,5-dimethylimidazol-2-ylidene-1-yl)). The Fe(II) center of 6 traps atmospheric dinitrogen reversibly to produce a dinuclear end-on N complex [Cp*Fe{κ -(C,C)- L′ }] (μ-η :η - N ) (7). Complex 6 also promotes C-H bond activation of thiophene, furan, benzothiophene, and benzofuran at room temperature. In these reactions, C-H bond cleavage occurred exclusively at the 2-position of the rings, generating Cp*Fe(L )(2-C H E) (8; E = S, 9; E = O) and Cp*Fe(L )(2-C H E) (10; E = S, 11; E = O), while C-H cleavage took place mainly at the 4-position in the case of pyridine. Coupling reactions between heteroarenes and catecholborane (HBcat) can be carried out by treatment of 6 with heteroarenes followed by the addition of excess HBcat, giving rise to 2-boryl-heteroarenes and the borohydride complex Cp*Fe(L )(H Bcat) (14). © 2008 American Chemical Society. R Mes iPr 5 Mes 2 Mes Mes 2 iPr iPr 2 iPr 1 1 iPr iPr iPr 5 5 3 2 2 6 2 2 2 3 2 2 2 4 3 8 5 2