Valence shell charge concentrations at pentacoordinate d(0) transition-metal centers: Non-VSEPR structures of Me(2)NhCl(3) and Me3NbCl2

摘要

The molecular structures of the monomeric, pentacoordinated methylchloroniobium(IV) compounds Me3NbCl2 and Me2NbCl3, have been determined by gas electron diffraction (GED) and density functional theory (I)FT) 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 C-3h geometry, in line with the results of quantum chemical calculations. Structure optimizations by DFT calculations and least-squares refinement to the GED data yield the following structural parameters (calcd/exptl; eq=equatorial; ax= axial; distances in angstrom, angles in degrees; average values in <> brackets): Me3NbCl2, in C-3v symmetry, Nb-Cl < 2.370 >/< 2.319(3)>, Nb-C 2.173/ 2.152(4), C-H < 1.096 >/1.124(5), - NbCH < 109.3 >/105.2(8), - C1NbC 92.2/93.3(2), - CNbC 119.9/119.7(l); Me2NbCl3, in C-2v, symmetry, Nb-Cl-ax 2.361/2.304(5), Nb-Cl q 2.321/2.288(9), Nb-C 2.180/2.135(9), C-H (1.094)/ 1.12(l), Cl xNbCl q 98.5/96.5(6), - CNbC 121.0/114(2), - c NbCH 108.9)/109(2). 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.