Effect of ligand addition and substitution on metal-metal multiple bonds: Direct electronic structure comparisons via gas phase photoelectron spectroscopy.

Abstract

Several series of complexes containing metal-metal and metal-heteroatom multiple bonds have been examined via gas-phase photoelectron spectroscopy to study the electronic structure effects resulting from simple ligand addition or substitution. Two types of complexes containing metal-metal multiple bonds form the basis of this study: complexes containing the M₂X₄P₄ core (where M = Mo, Re; X = halogen, Me; and P₄ = (PMe₃)₄ or [R₂P(CH₂)ₓPR₂]₂) and the dinuclear cyclopentadienyl metal carbonyl complexes [CpM(CO)ₓ]₂ (where M = Cr, Mo, W and x = 2,3). In the M₂X₄P₄ dimers two basic substitutions were examined. One involved changing the degree of rotation between the metal centers by varying the chelating phosphine ligand bridging the two centers. The other substitution involved varying the X ligand among Cl, Br, I, and Me on Mo₂ and Re₂ complexes with a monodentate phosphine, PMe₃. The synthesis and characterization of Re₂I₄ (PMe₃)₄, a previously unreported member of this series, is also described. For both of these substitutions, only very small changes were seen in the energy of the ionizations from orbitals involved in the metal-metal multiple bond. In the chelating phosphine complexes, changes in bandshape of the metal-based ionizations can be explained through the reduction in symmetry caused by placing the metals in a "staggered" configuration. For the monodentate phosphine complexes, the direction of the energy shifts which do occur indicate that changes in π-donor capability of the X ligand affect the metal-metal bonding to a greater extent than the changes in σ-donor capability. The effects of ligand addition to a metal-metal multiple bond (as in (CpM(CO)₂)₂) were examined, first by addition of one CO/metal, and then by insertion of a heteroatom into the multiple bond. In both cases, substantial energy shifts occur in related metal-based ionizations upon ligand addition. With CO addition, the M-M σ ionization is destabilized by ca. 1-2 eV, correlating with the longer M-M distances and increased reactivity in the (CpM(CO)₃)₂ systems. With the insertion of a chalcogen (S,Se) atom into the M-M triple bond of (CpCr(CO)₂)₂, the π-donor capability of the chalcogen results in M-heteroatom multiple bond character, though the PE spectra suggest that a full triple bond is not present.