MOLECULAR-STRUCTURE OF PENTAMETHYLANTIMONY BY GAS ELECTRON-DIFFRACTION - STRUCTURE AND BONDING IN SB(CH3)5 AND BI(CH3)5 STUDIED BY ABINITIO MO CALCULATIONS

The electron diffraction pattern of Sb(CH3)5 recorded with an all-glass inlet system at room temperature confirms the trigonal bipyramidal structure and yields the bond distances Sb-C(eq) = 214.0(5) pm and Sb-C(ax) = 226.4(11) pm. C-13 NMR spectra in CD2Cl2 contain only one line which remains sharp down to -90-degrees-C. The molecule presumably undergoes Berry pseudorotation over a square pyramidal transition state which leads to rapid exchange of axial and equatorial methyl groups. Trigonal bipyramidal and square pyramidal models of Sb(CH3)5 were optimized at the SCF MO level. Electron correlation was included with the modified coupled pair functional (MCPF) method. The calculations at this level suggest that the energy of the trigonal bipyramidal configuration is 7.1 kJ mol-1 below the energy of the square pyramidal configuration. Similar calculations on the unknown compound Bi(CH3)5 suggest that this molecule too would have an trigonal bipyramidal equilibrium configuration.