A comparison of the influences of alkoxide and thiolate ligands on the electronic structure and reactivity of molybdenum(3+) and tungsten(3+) complexes. Preparation and structures of M-2((OBu)-Bu-t)(2)((SBu)-Bu-t)(4), [Mo((SBu)-Bu-t)(3)((NO)](2), and W((SBu)-Bu-t)(3)(NO)(py)

M-2((OBu)-Bu-t)(6) compounds (M = Mo, W) react in hydrocarbon solvents with an excess of (BuSH)-Bu-t to give M-2((OBu)-Bu-t)(2)((SBu)-Bu-t)(4), red, a ir- and temperature-sensitive compounds. H-1 NMR studies reveal the equilib rium M-2((OBu)-Bu-t)(6) + 4(t)BuSH reversible arrowM(2)((OBu)-Bu-t)(2)((SBu )-Bu-t)(4) + 4(t)BuOH proceeds to the right slowly at 22 degreesC. The inte rmediates M-2((OBu)-Bu-t)(4)((SBu)-Bu-t)(2), M-2((OBu)-Bu-t)(3)((SBu)-Bu-t) (3), and M-2((OBu)-Bu-t)(5)((SBu)-Bu-t) have been detected. The equilibrium constants show the M-(OBu)-Bu-t bonds to be enthalpically favored over the M-(SBu)-Bu-t bonds. In contrast to the M-2((OBu)-Bu-t)(6) compounds, M-2(( OBu)-Bu-t)(2)((SBu)-Bu-t)(4) compounds are inert with respect to the additi on of CO, CO2, ethyne, (BuC)-Bu-t=CH, MeC=N, and PhC=N. Addition of an exce ss of (BuSH)-Bu-t to a hydrocarbon solution of W-2((OBu)-Bu-t)(6)(mu -CO) l eads to the rapid expulsion of CO and subsequent formation of W-2((OBu)-Bu- t)(2)((SBu)-Bu-t)(4). Addition of an excess of tBuSH to hydrocarbon solutio ns of [Mo((OBu)-Bu-t)(3)(NO)](2) and W((OBu)-Bu-t)(3)(NO)(py) gives the str ucturally related compounds [Mo((SBu)-Bu-t)(3)(NO)](2) and W((SBu)-Bu-t)(3) (NO)(py), with linear M-N-O moieties and five-coordinate metal atoms. The v alues of v(NO) are higher in the related thiolate compounds than in their a lkoxide counterparts. The bonding in the model compounds M-2(EH)(6), M-2(OH )(2)(EH)(4), (HE)(3)M=CMe, and W(EH)(3)(NO)(NH3) and the fragments M(EH)(3) , where M = Mo or W and E = O or S, has been examined by DFT B3LYP calculat ions employing various basis sets including polarization functions for O an d S and two different core potentials, LANL2 and relativistic CEP. BLYP cal culations were done with ZORA relativistic terms using ADF 2000. The calcul ations, irrespective of the method used, indicate that the M-O bonds are mo re ionic than the M-S bonds and that E p pi to M d pi bonding is more impor tant for E = O. The latter raises the M-M pi orbital energies by ca. 1 eV f or M-2(OH)(6) relative to M-2(SH)(6). For M(EH)(3) fragments, the metal d(x z),d(yx) orbitals are destabilized by OH p pi bonding, and in W(EH)(3)(NO)( NH3) the O p pi to M d pi donation enhances W d pi to NO pi* back-bonding. Estimates of the bond strengths for the M=M in M-2(EH)(6) compounds and M=C in (EH)(3)M=CMe have been obtained. The stronger pi donation of the alkoxi de ligands is proposed to enhance backbonding to the pi* orbitals of alkyne s and nitriles and facilitate their reductive cleavage, a reaction that is not observed for their thiolate counterpart.