The chemistry of monoanionic carbaborane ligands. Synthesis, and molecular and electronic structure of [3,3,3-(CO)₃-4-SMe₂-3,1,2-MnC₂B₉H₁₀], and order-of-magnitude improved structure of (η-C₅H₅)Mn(CO)₃

The synthesis, and spectroscopic and structural characterisation of a transition metal complex of, formally, a monoanionic carbaborane ligand are described. Two molecules of [3,3,3-(CO)₃-4-SMe₂-3,1,2-MnC₂B₉H₁₀] crystallise in the triclinic space group P1, with a 7.154(4), b 8.7890(21), c 13.366(3) Å, α 91.438(19), β 101.21(3), γ 110.69(3)°, at 185 ± 1 K. R = 0.0352 for 3859 observed reflections. The carbamanganaborane has an essentially icosahedral polyhedral geometry, and the pendant SMe₂ unit is oriented such that the S lone pair...H(1)^δ+ interaction is maximised. The average OCMnCO angle is 89.91°. A redetermination of the structure of the known species CpMn(CO)₃ affords molecular parameters of high precision. Space group P2₁/a with a 11.941(7), b 6.981(5), c 10.798(7) Å, β 117.97(5)°, Z = 4, R = 0.0418 for 1960 significant reflections measured at 185 ± 1 K. In this compound OCMnCO is wider, average 92.14°. Charge iterated EHMO calculations suggest that the anionic carbaborane ligand may be represented by a form in which a charge of -1.5 e on the five facial atoms is partially offset by a charge of +0.3 e on the pendant S atom. EHMO/FMO calculations confirm that the carbaborane is a better electron donor than the Cp ligand to the {Mn(CO)₃} unit, and indicate that the additional electron density on Mn resides in fragment orbitals that are antibonding between C and O. In particular, CO → Mn σ donation is restricted in the carbaborane compound relative to the Cp compound. Consistent with the results of these calculations, ν(CO) values of the former compound are measurably lower than those of the latter species.