Mono- and Dicoordinate Germanium(0) as a Four-Electron Donor.

We report the synthesis and isolation of molecular iron germanide motifs in the stoichiometry of Fe3 Ge and Fe2 Ge, which are stabilized by the coordination of N-heterocyclic carbene (NHC) and carbon monoxide (CO) ligands. NHCiPr2Me2 ⋅Ge[Fe(CO)4 ][Fe2 (CO)8 ] (NHCiPr2Me2 =1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), a germanium(0) ligand with just one NHC as an auxiliary ligand, assumes a bridging coordination mode between a mononuclear [Fe(CO)4 ] and a dinuclear [(CO)4 FeFe(CO)4 ] unit with rapid exchange between the two binding modes in solution. The electronic structure of this species is analysed by NBO and ELF calculations with DFT methods, as well as the mechanistic details of its fluxional coordination behaviour. Treatment of NHCiPr2Me2 ⋅Ge[Fe(CO)4 ] [Fe2 (CO)8 ] with the sterically less demanding NHCMe4 (NHCMe4 =1,3,4,5-tetramethylimidazol-2-ylidene) leads to a dinuclear iron complex (NHCMe4 )2 Ge[Fe(CO)4 ]2 that contains a bridging germylone ligand with two stabilizing NHC equivalents.