Molecular Structures of Pentamethylarsenic(V) and Trimethyldichloroarsenic(V) by Gas Electron Diffraction and ab Initio Calculations: Molecular Mechanics Calculations on Pentamethylarsenic(V), Pentaphenylarsenic(V), and Related Compounds

The molecular structures of AsMe 5 and AsMe 3 Cl 2 (Me = CH 3 ) have been determined by gas electron diffraction and Hartree−Fock computations. Both molecules are found to have trigonal bipyramidal arrangements of the heavy atom skeleton. The bond distances ( r a ) for AsMe 5 are As−C eq = 197.5(3) pm and As−C ax = 207.3(4) pm; for AsMe 3 Cl 2, As−C eq = 192.5(2) pm and As−Cl ax = 234.9(3) pm. The coordination geometries and dimensions are compared with those of related molecules. Molecular mechanics calculations on the pentamethyl and pentaphenyl derivatives of P, As, Sb, and Bi reproduce the observed coordination geometries and indicate that the square pyramidal equilibrium structures of SbPh 5 and BiPh 5 are stabilized by Coulombic interactions between the ligands.