10-VERTEX POLYHEDRAL RHODADICARBABORANE CHEMISTRY - MOLECULAR-STRUCTURE, NMR PROPERTIES, AND CLUSTER DYNAMIC BEHAVIOR OF [2,2,2-(PPH3)2H-CLOSO-2,1,6-RHC2B7H9]

The compound previously described as [6,6,6-(PPh3)2H-closo-6,2,3-RhC2B 7H9] is shown by detailed multielement NMR studies and single-crystal X-ray diffraction analysis to be its isomer 12,2,2-(PPh3)2H-closo-2,1, 6-RhC2B7H9]. Crystals (from pentane-diethyl ether) were monoclinic, sp ace group P2(1)/n, with Z = 4, and with a = 1222.2(1) pm, b = 1 598.7( 2) pm, c = 1836.7(1) pm, and beta = 91.86(1)-degrees. The structure wa s refined to R(R(W)) 0.0295(0.0414) using 6 819 observed data [I > 2.0 sigma(I)] out of 8 541 collected at 200 K. The {RhC2B7} cluster struc ture is to a first approximation classically closo, with the metal and two carbon atoms in adjacent 2, 1, and 6 positions, respectively. Bot h enantiomers exist in the unit cell, and variable temperature NMR stu dies show that the molecule is fluxional (DELTAG355not-equal approxima tely 60 kJ mol-1) between enantiomers via a diamond-square diamond clu ster rearrangement.