SUBSTRATE UPTAKE AND ACTIVATION BY DIMOLYBDENUM AND DITUNGSTEN HEXAALLKOXIDES - FACTORS INFLUENCING THE CLEAVAGE OF C-X MULTIPLE BONDS IN THE REACTIONS BETWEEN [MO-2(OR)(6)] (R=TBU OR CH(2)TBU) AND AR2C=S, ET(2)NC-N, AND P(NBU)(3)

The reaction between diaryl-thiones and [Mo-2(OCH(2)tBu)(6)] in hydro- carbon solvents yielded [Mo-2(OCH(2)tBu)(6)-(mu-S)( = CAr2)] by cleava ge of the C = S double bond. The Lewis base adduct [Mo-2 (OCH(2)tBu)(6 )(mu-S) ( = CPh(2)) (PMe(3))] (2) has been crystallographically charac terized; it contains six- and five-coordinate Mo atoms linked through mu-S and mu-OR groups. The rate of the cleavage of the C = S bond has been studied by variable-temperature H-1 NMR in [D-8]toluene. PL Hamme tt plot shows that both electron-donating and electron-releasing subst ituents in the aryl groups enhance the rate relative to Ph(2)C = S. Th e activation parameters for cleavage of the C = S bonds in Ph(2)C = S, (p-MeOC(6)H(4))(2)C = S, and (m-CF3C6H4)(2)C = S exhibit essentially identical values for Delta S double dagger, while Delta H double dagge r is some-what smaller for the thiones bearing electron-donating or -w ithdrawing substituents. We also studied the reversible binding of Et( 2)NC = N to [Mo-2(OtBu)(6)] as a model system wherein the C = N bond i s reduced but not cleaved and the (M = M)(6+) center oxidized. Finally , the reversible binding of P(nBu)(3) to [Mo-2(OCH(2)tBu)(6)], wherein the product exhibits no net oxidation of the metal centers, has been similarly studied. The activation parameters for these three classes o f reactions are compared with previous studies of the reversible bindi ng of CN- [M(2)(OR)(6)] compounds and the reaction 2[W-2(OiPr)(6)] rev ersible arrow [W-4(OiPr)(12)]. We propose that the activation paramete rs associated with C = S cleavage are associated with the formation of a mu-eta(1),eta(2)-S-CAr2 reactive complex similar to others describe d elsewhere,([23]) and that they result from a) the bimolecular nature of the reaction and b)reorganizational energy. The greater facility o f C-X multiple-bond cleavage in reactions involving [W-2(OR)(6)] compo unds relative to [Mo-2(OR)(6)] compounds, the substituent effects, and the dependence on R are discussed in terms of the orbital energy matc h between the M-M and C-X pi and pi orbitals, and an analogy is made with neutral Diels-Alder cycloaddition reactions.