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-(N Me2)(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 we re determined by single crystal X-ray diffraction and 1 represents the firs t structurally characterised example of a 2,1,12-MC2B9 metallacarborane. Co mparison of B-11 NMR data of the tantalum complexes, along with the isomeri c [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 {B H} vertex, influences significantly the boron NMR shifts of the neighbourin g and antipodal cage atoms. Based on this observation the assignments of th e reported peaks in the boron NMR data for the seven isomers of (eta(5)-C5H 5)CoC2B9H11 are tentatively predicted.