The chemistry of monoanionic carbaborane ligands. Synthesis and structure of compounds with carbonyl and cycloocta-1,5-diene ligands additionally bound to metal atoms, including a cationic carbametallaborane

Reaction of [(ML₂Cl)₂] with Tl[9-SMe₂-nido-7.8-C₂B₉H₁₀] affords the new compounds [3,3-L₂-4-SMe₂-closo-3,1,2-MC₂B ₉H₁₀]. Compounds 1 (M = Rh, L = CO), 2 (M = Rh, L₂ = η²,η²-1,5-C₈H₁₂) and 3 (M = Pd, L₂ = η², σ-5OMe-C₈H₁₂) were characterised by microanalysis, multinuclear NMR spectroscopy and, in the case of 1 and 2, single-crystal X-ray diffraction studies. The experimentally determined conformations of the ML₂ fragments relative to the carbaborane ligands in 1 and 2 are in broad agreement with those predicted by analysis of the results of extended Hückel molecular orbital (EHMO) calculations on model compounds, especially in the case of 2. Reaction of 3 with HBF₄ in diethyl ether affords the cationic heteroborane [3-(η²,η²-1,5-C₈H ₁₂)-4-SMe₂-closo-3,1,2-PdC₂B₉H ₁₀]⁺ 4⁺. Crystallographic study of 4⁺BF₄^- reveals a molecular conformation substantially influenced by H ⋯ F inter-ion contacts. Comparative EHMO calculations on 2 and 4⁺ imply that the majority of the additional positive charge in the latter is not localised on the pendant sulfur atom alone, but rather is delocalised over the 12 cluster vertices and the atoms directly bonded to them.