Two diastereomeric pairs of nickel bis(C-monomethyldicarbollide) complexes, derived from the racemic [nido-7-CH3-7,8-C2B9H11](-) anion, have been synthesized and characterized by NMR spectroscopy and single-crystal X-ray diffraction analysis. Neutral (dd/ll) [1(2),1'(2')-Me-2-closo-3,1,2-(NiC2B9H10)-C-IV-{3:3'}-closo-3',1',2'-(Ni C2B9H10)-C-IV] 1 and (meso) [1,2'-Me-2-closo-3,1,2-(NiC2B9H10)-C-IV-{3:3'}-closo-3',1',2'-(NiC2B9H10 )-C-IV] 2 adopt typical cisoid conformations in the solid state. The temperature-dependent B-11 and H-1 NMR spectra of 2 indicate that the energy barrier to the interconversion of racemic rotational isomers is 66.5 +/- 2 kJ/mol. In the solid state, the [NMe4](+) salts of the (dd/ll) [1(2),1'(2')-Me-2-closo-3,1,2-(NiC2B9H10)-C-III-{3:3'}-closo-3',1',2'-(N iC2B9H10)-C-III](-) anion NMe4 center dot 3 and the (meso) [1,2'-Me-2-closo-3,1,2-Ni-III C2B9H10-{3:3'}-closo-3',1',2'-(NiC2B9H10)-C-III](-) anion NMe4 center dot 4 adopt gauche and transoid configurations, respectively, with transoid methyl substituents in both cases.