First structural characterisation of a 2,1,12-MC2B9 metallacarborane, [2,2,2-(NMe2)3-closo-2,1,12-TaC2B9H11].: Trends in boron NMR shifts on replacing a {BH} vertex with a metal {MLn} vertex in icosahedral carboranes

Reactions of M(NMe2)(5) (M = Ta or Nb) with nido-2,9-C2B9H13 and the salt [Me3NH][nido-7,9-C2B9H12] gave the isomeric dicarbollide complexes [2,2,2-(NMe2)(3)-closo-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 1 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 B-11 NMR data of the tantalum complexes, along with the isomeric [3,3,3-(NMe2)(3)-closo-3,1,2-TaC2B9H11] 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 (eta(5)-C5H5)CoC2B9H11 are tentatively predicted.