Gas-Phase Ultraviolet Photoelectron Studies of Octahedral Tris(pyrazol-1-yl) borate Complexes of Some Divalent Transition Metals

This paper reports the results of a study of the electronic structure of both early and late-transition-metal 3d hydrotris(pyrazol-1 -yl)borate complexes by combined He I and He II photoelectron spectroscopy. This is the first investigation of a series of analogous complexes with divalent transition metals spanning from Fe to Zn in an identical octahedral array. No gas-phase photoelectron spectra of octahedral Ni, Cu, and Zn complexes have ever been reported. In the case of Fe and Co complexes, there is evidence that the energy of the final states produced upon ionization of electrons from metal 3d subshells can be accounted for by an electrostatic model mediated by the usual Racah and ligand field parameters. As a natural consequence, the ionization energies of ligand-based molecular orbitals are no longer affected by the nature of the central metal atom. In the series of complexes from Fe to Cu, the ionization energies of the 3d subshells increase smoothly. This trend, probably, causes in the Ni and Cu complexes a better energy matching between metal 3d orbitals and combinations of ring nitrogen lone pairs of appropriate symmetry and, thence, leads to predominant, covalent metal-to-ligand bonding. A localized bonding model, reminiscent of that adopted for metallocenes, provides a good rationale for the spectra.