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, air- and temperature-sensitive compounds. H-1 NMR studies reveal the equilibrium 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 intermediates 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 addition of CO, CO2, ethyne, (BuC)-Bu-t=CH, MeC=N, and PhC=N. Addition of an excess of (BuSH)-Bu-t to a hydrocarbon solution of W-2((OBu)-Bu-t)(6)(mu -CO) leads 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 solutions of [Mo((OBu)-Bu-t)(3)(NO)](2) and W((OBu)-Bu-t)(3)(NO)(py) gives the structurally 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 values of v(NO) are higher in the related thiolate compounds than in their alkoxide 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 calculations employing various basis sets including polarization functions for O and S and two different core potentials, LANL2 and relativistic CEP. BLYP calculations were done with ZORA relativistic terms using ADF 2000. The calculations, irrespective of the method used, indicate that the M-O bonds are more ionic than the M-S bonds and that E p pi to M d pi bonding is more important for E = O. The latter raises the M-M pi orbital energies by ca. 1 eV for M-2(OH)(6) relative to M-2(SH)(6). For M(EH)(3) fragments, the metal d(xz),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 alkoxide ligands is proposed to enhance backbonding to the pi* orbitals of alkynes and nitriles and facilitate their reductive cleavage, a reaction that is not observed for their thiolate counterpart.