Structure and reactivity of some quadruply bonded dimolybdenum complexes

Abstract

Six quadruply bonded dimolybdenum(II) complexes of the type Mo(,2)X(,4)(PP)(,2), in which X = Cl or Br and PP = a bridging diphosphine ligand, have been synthesized and structurally characterized. The rotational conformation of these complexes varies from strictly eclipsed to almost completely staggered, causing the strength of the (delta) bond to change. These six complexes, together with five others that have been previously studied structurally, are used to investigate the dependence of Mo-Mo quadruple bond length and (delta)-(delta)* electronic transition energy on internal torsion angle (chi) (the angle of twist away from the perfectly eclipsed conformation). The Mo-Mo distance is found to vary as an inverse linear function of cos(2(chi)). The extrapolated Mo-Mo triple bond length for a totally staggered structure ((chi) = 45(DEGREES), cos(2(chi)) = 0) is 2.194 (ANGSTROM). The (delta)-(delta)* transition energy is shown to have a more complex dependence on cos(2(chi)), extrapolating to a value of 11.9 (+OR-) 0.4 x 10('3) cm('-1) at (chi) = 45(DEGREES) instead of vanishing. It is proposed that this "residual" energy is principally due to the difference between correlation energies in the ground ((sigma)('2)(pi)('4)(delta)('2)) and excited ((sigma)('2)(pi)('4)(delta)(delta)*) electron configurations. The manner in which these relationships are affected by the different steric and electronic parameters of the PP ligands has also been studied by the preparation and structural characterization of a triply bonded dirhenium(II) Re(,2)X(,4)(PP)(,2) compound. In addition, the susceptibility of several quadruply bonded dimers of the types Mo(,2)X(,4)L(,4), where L is a monodentate ligand, and Mo(,2)X(,4)(LL)(,2), where LL is a bidentate ligand, toward oxidative addition has been investigated. In general, Mo(,2)X(,4)(LL)(,2) species undergo oxidative addition reactions with organic disulfides (RSSR) to give molybdenum(III) dimers of the type Mo(,2)((mu)-SR)(,2)X(,4)(LL)(,2), while Mo(,2)X(,4)L(,4) compounds fail to react under analogous conditions. The new Mo(III) dimers have edge-sharing bioctahedral structures and Mo-Mo single or double bonds.