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 cleavage 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 characterized; 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 Hammett plot shows that both electron-donating and electron-releasing substituents in the aryl groups enhance the rate relative to Ph(2)C = S. The 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 dagger is some-what smaller for the thiones bearing electron-donating or -withdrawing 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 is 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 of reactions are compared with previous studies of the reversible binding of CN- [M(2)(OR)(6)] compounds and the reaction 2[W-2(OiPr)(6)] reversible arrow [W-4(OiPr)(12)]. We propose that the activation parameters associated with C = S cleavage are associated with the formation of a mu-eta(1),eta(2)-S-CAr2 reactive complex similar to others described elsewhere,([23]) and that they result from a) the bimolecular nature of the reaction and b)reorganizational energy. The greater facility of C-X multiple-bond cleavage in reactions involving [W-2(OR)(6)] compounds relative to [Mo-2(OR)(6)] compounds, the substituent effects, and the dependence on R are discussed in terms of the orbital energy match between the M-M and C-X pi and pi* orbitals, and an analogy is made with neutral Diels-Alder cycloaddition reactions.