AB INITIO CALCULATIONS OF METAL-METAL COMPLEXES USING RELATIVISTIC EFFECTIVE CORE POTENTIALS

Abstract

Metal-metal complexes containing quadruple bonds have attracted much interest in inorganic chemistry. The two types studied were molybdenum and rhenium complexes. Previous theoretical, mainly local density functional, and experimental studies have disagreed on photo-electron and ultraviolet spectroscopic assignments. This study calculated the first few peaks in both spectra for both complexes. Atomic basis sets were optimized for molybdenum, rhenium, chlorine, oxygen, and phosphorus for use with Christiansen relativistic effective core potentials. Hartree-Fock and MCSCF calculations for these complexes were performed on the ground state and several excited states. Spin-orbit splittings of these states were found using a double group spin-orbit configuration interaction (CI) program. The known metal-metal quadruple bond was investigated and, as previously shown, was not found at the Hartree-Fock level. Corresponding calculations at the MCSCF level on the ground state of these complexes showed this quadruple bond. The criteria chosen for the description of the bonds were the Mullikan populations, the molecular orbital and CI coefficients, natural orbital populations, and contour plots. The lowest excited state for the rhenium complexes was found to be a $^{3}A_{1}, \delta-\delta^{\ast}$ state with spin-orbit coupling of the order of $50 cm^{-1}$. Calculations have also been completed on two spectroscopically important excited states, the $\delta-\delta^{\ast}$ singlet state and a ligand-metal charge transfer state. Monovalent complex cations were studied for several of the complexes and compared to the photoelectron spectra. For the dimolybdenum complexes, the effects of different strengths of $\pi$-donating ligands on the metal-metal bond were studied.