New supraicosahedral metallacarboranes. The synthesis and molecular structures of 4-dppe-4,1,6-closo-NiC₂B₁₀H₁₂ and [4-(η-C₃H₅)-4-(CO)₂-4,1,6-closo-MoC ₂B₁₀H₁₂]^-

The new 13-vertex nickelacarborane 4-dppe-4,1,6-closo-NiC₂B₁₀H₁₂ and molybdacarborane [4-(?-C₃H₅)-4-(CO)₂-4,1,6-closo-MoC ₂B₁₀H₁₂]^- have been prepared. The structure of the CH₂Cl₂ solvate of the nickel species has been studied as two crystalline modifications. The molybdacarborane was also studied crystallographically as its [PhCH₂NMe₃]⁺ salt. The differing trans influences of C and B atoms in the metalla-bonded C₂B₄ carborane ligand face are used to rationalise the difference in Ni-P distances in the nickelacarborane and the orientation of the {Mo(C₃H₅)(CO)₂} fragment in the molybdacarborane. Both metallacarboranes display fluxionality in solution at room temperature which can be arrested by cooling. An activation energy for the presumed double diamond-square-diamond fluxional process is calculated to be ca. 47 kJ mol^-1 by analysis of the ¹H and ³¹P NMR spectra of the nickelacarborane. For the molybdacarborane the equivalent activation energy is ca. 40 kJ mol^-1 from the ¹H spectrum. © 2002 Elsevier Science B.V. All rights reserved.