  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.
