Valence Shell Charge Concentrations at Pentacoordinate d0 Transition Metal Centers: Non-VSEPR Structures of Me2NbCl3 and Me3NbCl2

The molecular structures of the monomeric, pentacoordinated ethylchloroniobium(V) compounds Me3NbCl2 and Me2NbCl3 have been determined by gas electron diffraction (GED)and Density Functional Theory (DFT) calculations, and, for Me3NbCl2, by single crystal X-ray diffraction. Each of the molecules is found to have a heavy-atom skeleton in the form of a trigonal bipyramid (TBP) with Cl atoms in the axial positions, in accord with their vibrational spectra. The TBP is somewhat distorted in the case of Me2NbCl3 with the two axial Nb-Cl bonds bent away from the equatorial, slightly shorter Nb-Cl bond. In the case of Me3NbCl2, moreover, the X-ray model suggests structural distortions away from the idealized C3h geometry, in line with the results of quantum chemical calculations. The electronic structures of Me2NbCl3 and Me3NbCl2 have been explored by rigorous analysis of both the wavefunction and the topology of the electron density, employing DFT calculations. Hence the structures of these compounds are shown to reflect repulsion between the Nb-C and Nb-Cl bonding electron density and charge concentrations induced by the methyl ligands in the valence shell of the Nb atom and arising mainly from use of Nb(4d) functions in the Nb-C bonds.