Ligand electronic effects in dimetal complexes: Developments toward low ionization energies and correlations with solution properties

The electronic effects of ligand substitution on paddlewheel-type metal-metal bonded systems with the general formula M2( L⌢L )4 have been studied using ultraviolet photoelectron spectroscopy (UPS) and computational methods. The primary focus was to determine the influence of solvent interaction in the solution phase, and to explore what ligand properties produce low ionization energies when combined with dimetal cores.; The first ionization energies (IE1) of two substituted dimetal systems, determined by UPS in the gas phase, were compared to the E1/2 values obtained by electrochemical oxidation potentials in solution phase. The first dimetal system, substituted Mo2(DPhF)4 (where DPhF is N, N'-diphenylformamidinate), has a linear correlation of E1/2 with IE1 with a slope of 0.41. This indicates a significant amount of solvent stabilization of the cation, with the effect increasing with electron withdrawing substituents. This is attributed to the removal of electron density from the metal centers, making axial coordination of an electron donating solvent favorable. A linear trend also was found for Rh2(DPhF)4 but with a E1/2:IE1 slope of 0.53, indicating less solvent effect than with the molybdenum analogues. A possibility for the difference in slopes of the molybdenum and rhodium may be due to pi-backbonding from the solvent (CH2Cl2) into the empty molybdenum pi* orbitals. These pi* orbitals are filled in the rhodium system, so stabilization of the cation would not be possible by this mechanism.; A molecule with IE1 lower than any element or chemically prepared molecule was found during this research. The factors contributing to low IE 1 of stable, closed-shell molecules were explored by varying metals down a period while retaining the same ligand, and by varying the ligand by geometry constraints and alkyl substitutions while keeping a similar metal core structure. The results indicate two factors contribute to the unusual properties of M2(hpp)4 (where M = Cr, Mo or W and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyramido[1,2- a]pyrimidine): (1) strong orbital overlap interactions of a ligand b2g orbital with the metal delta orbital, and (2) the relative inability of the hpp anion to stabilize negative charge.