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

He I and He II photoelectron (PE) spectra of Me3NbCl2, Me2NbCl3 and Me3TaCl 2 are presented and assigned. Density functional calculations give trigonal bipyramidal structures for the Nb complexes, with the axial positions occu pied by Cl ligands. Calculated ionisation energies (IE) for Me3NbCl2, Me2Nb Cl3 and Me3TaCl2 are in good agreement with the experimental values and the molecular orbital compositions can be reconciled with the intensity change s in the PE spectra. The axial interactions are best described by a 3-centr e 4-electron bond, which accounts for the longer Nb-Cl distances predicted for these bond lengths. That the orbitals involved in Ta-ligand bonding wit h a Ta 6s contribution have higher IE's relative to their Nb counterparts m ay be due to relativistic stabilisation of the Ta 6s orbital. Comparisons w ith the main group analogue Me3SbCl2 support Elbel's assignment of the PE s pectrum of the latter compound, where there is a reversal of the IE orderin g between the transition metal and the main group compounds. For Sb the hal ide orbitals constitute the highest lying set, whereas with the transition metal compounds the metal-carbon bonding orbitals provide the highest occup ied molecular orbitals. No evidence is found in the calculations for a sign ificant contribution to the bonding from Nb 5p orbitals. The large angles s ubtended by the Cl atoms at the metal are attributed to their negative char ge and interligand repulsion.