Transition metal dicarbollide complexes: Synthesis, molecular, crystal and electronic 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 characterised by a structural study) reacted with the carborane nido-C₂B₉H₁₃ to eliminate two equivalents of HNMe₂ and generate the dicarbollide half-sandwich tris(dimethylamide) complexes [M(C₂B₉H₁₁)(NMe₂)₃] (M = Nb 3 or Ta 4). The crystal structures of isomorphous 3 and 4 have been determined and reveal two NMe₂ ligands in a vertical orientation and the third one in a horizontal orientation with respect to the η⁵-co-ordinated face of the C₂B₉H₁₁ ligand. The electronic factors responsible for the amide ligand orientations in these complexes are explored using qualitative MO arguments. Complexes 3 and 4 reacted with CO₂ and CS₂ to yield the tris(carbamate) [M(C₂B₉H₁₁)(O₂CNMe₂) 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 and 7 show two (dithio)carbamate ligands in horizontal and one in vertical orientation, demonstrating the similarity between the σ,π-donor frontier orbitals of the ligands NMe₂ and X₂CNMe₂ in 4 and 6 or 7 respectively.