Aspects of transition metal pentacoordination: investigation of the structure and bonding of Me3NbCl2, Me3TaCl2 and Me2NbCl3 by photoelectron spectroscopy and density functional theory calculations

Abstract

He I and He II photoelectron (PE) spectra of Me₃NbCl₂, Me₂NbCl₃ and Me₃TaCl₂ are presented and assigned. Density functional calculations give trigonal bipyramidal structures for the Nb complexes, with the axial positions occupied by Cl ligands. Calculated ionisation energies (IE) for Me₃NbCl₂, Me₂NbCl₃ and Me₃TaCl₂ are in good agreement with the experimental values and the molecular orbital compositions can be reconciled with the intensity changes in the PE spectra. The axial interactions are best described by a 3-centre 4-electron bond, which accounts for the longer Nb–Cl distances predicted for these bond lengths. That the orbitals involved in Ta–ligand bonding with a Ta 6s contribution have higher IE’s relative to their Nb counterparts may be due to relativistic stabilisation of the Ta 6s orbital. Comparisons with the main group analogue Me₃SbCl₂ support Elbel’s assignment of the PE spectrum of the latter compound, where there is a reversal of the IE ordering between the transition metal and the main group compounds. For Sb the halide orbitals constitute the highest lying set, whereas with the transition metal compounds the metal–carbon bonding orbitals provide the highest occupied molecular orbitals. No evidence is found in the calculations for a significant contribution to the bonding from Nb 5p orbitals. The large angles subtended by the Cl atoms at the metal are attributed to their negative charge and interligand repulsion.