Transition metal dicarbollide complexes: synthesis, molecular, crystal andelectronic structures of [M(C2B9H11)(NMe2)(3)] (M = Nb or Ta) and their insertion reactions with CO2 and CS2

The homoleptic amides [M(NMe2)(5)] (M=Nb 1 or Ta 2; the latter is character ised by a structural study) reacted with the carborane nido-C2B9H13 to elim inate two equivalents of HNMe2 and generate the dicarbollide half-sandwich tris(dimethylamide) complexes [M(C2B9H11)(NMe2)(3)] (M=Nb 3 or Ta 4). The c rystal structures of isomorphous 3 and 4 have been determined and reveal tw o NMe2 ligands in a vertical orientation and the third one in a horizontal orientation with respect to the eta(5)-co-ordinated face of the C2B9H11 lig and. The electronic factors responsible for the amide ligand orientations i n these complexes are explored using qualitative MO arguments. Complexes 3 and 4 reacted with CO2 and CS2 to yield the tris(carbamate) [M(C2B9H11)(O2C NMe2)(3)] (M=Nb 5 or Ta 7) and tris(dithiocarbamate) [M(C2B9H11)(S2CNMe2)(3 )] (M=Nb 6 or Ta 8) complexes, respectively. The crystal structures of 6 an d 7 show two (dithio)carbamate ligands in horizontal and one in vertical or ientation, demonstrating the similarity between the sigma,pi-donor frontier orbitals of the ligands NMe2 and X2CNMe2 in 4 and 6 or 7 respectively.