First structural characterisation of a 2,1,12-MC₂B₉ metallacarborane, [2,2,2-(NMe₂)₃-closo-2,1,12-TaC₂B ₉H₁₁]. Trends in boron NMR shifts on replacing a {BH} vertex with a metal {ML_n} vertex in icosahedral carboranes

Reactions of M(NMe2)5 (M = Ta or Nb) with nido-2,9-C₂B9H₁₃ and the salt [Me3NH][;Y/o-7,9-C2B9H₁₂] gave the isomeric dicarbollide complexes [2,2,2-(NMe₂)₃-c/ajo-2,1,12-MC2B9H₁₁] (M = Ta 1, Nb 4) and [2,2,2-(NMe₂)₃-closo-2,1,7-MC2B9H₁₁] (M = Ta 2, Nb 5) respectively. The structures of l and 2 were determined by single crystal X-ray diffraction and 1 represents the first structurally characterised example of a 2,1,12-MC₂B₉ metallacarborane. Comparison of ¹¹B NMR data of the tantalum complexes, along with the isomeric [3,3,3-(NMe2)3-c/o5o-3,1,2TaC₂B₉H₁₁] ₃, with that of 1,2-, 1,7- and 1,12-C₂B₁₀H₁₂, reveals that the metal vertex {Ta(NMe₂)₃}, on replacing a {BH} vertex, influences significantly the boron NMR shifts of the neighbouring and antipodal cage atoms. Based on this observation the assignments of the reported peaks in the boron NMR data for the seven isomers of (n₅-CjH₅)CoC₂B₉H_n are tentatively predicted.