The cage-alkylated metallacarborane complex [4,4,4-(NMe2)(3)-3- Me-4,1,2-closo-TaC2B9H10] 8, is obtained as the only product from the reaction of Ta(NMe2)(5) with nido-11-Me-2,7-C2B9H12 4, which is prone to skeletal rearrangement under basic conditions. That no rearrangement occurs indicates that the hydroamminolysis reaction is a clean synthetic method. The isomeric metallacarboranes [3,3,3-(NMe2)(3)-4-Me-3,1,2-closo- TaC2B9H10] 9 and [2,2,2-(NMe2)(3)-3-Me-2,1,7-closo-TaC2B9H10] 10 are produced from the reaction of Ta(NMe2)(5) with [Me3NH][nido-9-Me-7,8-C2B9H11] 5 and [Me3NH][nido-8-Me-7,9- C2B9H11] 6 respectively. Identities of the metallacarboranes 8, 9 and 10 are deduced by detailed multinuclear (B-11, C-13 and H-1) NMR spectroscopy. These geometries are supported by boron NMR shift predictions based on observed shifts of the known non-methylated analogues [3,3,3-(NMe2)(3)-closo-3,1,2- TaC2B9H10] 1 and [3,3,3-(NMe2)(3)-closo-3,1,2-TaC2B9H10] 2 and calculated shifts (GIAO-B3LYP/6-311G*//MP2/6-31G*) of the B- methyl carboranes, closo-3-Me-1,2-C2B10H11 11, 4-Me-1,2- C2B10H11 12 and 2-Me-1,7-C2B10H11 13. The molecular structure of 8 has been determined by X-ray diffraction.