The oxo-Mo(IV) complexes LMoO(S(2)PR(2)-S,S') [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate; R = Me, Et, Pr(i)(), Ph] were prepared by reacting MoO(S(2)PR(2))(2) and KL in refluxing toluene. The dioxo-Mo(VI) complexes cis-LMoO(2)(S(2)PR(2)-S) (R = Pr(i)(), Ph) were prepared by oxidation of the oxo-Mo(IV) complexes or by reaction of LMoO(2)Cl with NaS(2)PR(2). Oxygen atom transfers from Me(2)SO to LMoO(S(2)PR(2)) were first-order with respect to Me(2)SO and complex; the overall second-order rate constants at 40 degrees C range from 9.0(1) x 10(-)(5) M(-)(1).s(-)(1) for LMoO(S(2)PMe(2)) to 2.08(5) x 10(-)(4) M(-)(1).s(-)(1) for LMoO(S(2)PPr(2)); activation parameters were in the ranges DeltaH() = 63(1) to 73(1) kJ.mol(-)(1), DeltaS() = -88(1) to -111(1) J.K(-)(1).mol(-)(1), and DeltaG() = 100(2) kJ.mol(-)(1) for LMoO(S(2)PMe(2)) to 98(2) kJ.mol(-)(1) for LMoO(S(2)PPr(2)). Oxygen atom transfer from pyridine N-oxide to LMoO(S(2)PPr(2)) was also second-order with a rate constant of 1.54(5) x 10(-)(3) M(-)(1).s(-)(1) at 40 degrees C, DeltaH() = 62(1) kJ.mol(-)(1), DeltaS() = -90(1) J.K(-)(1).mol(-)(1), and DeltaG() = 90(1) kJ.mol(-)(1). The second-order rate laws and large negative entropies of activation are consistent with associative mechanisms for the above reactions. Oxygen atom transfer from LMoO(2)(S(2)PPr(2)) to PPh(3) was first-order with respect to reactants, with an overall second-order rate constant of 2.5(3) x 10(-)(4) M(-)(1).s(-)(1) at 30 degrees C. In toluene at 40 degrees C, all the above complexes catalyzed the oxidation of PPh(3) by Me(2)SO, with turnover rates of ca. 0.9 mol of PPh(3)/(mol of catalyst/h). Reduction of LMoO(2)(S(2)PR(2)) by SH(-) led to the generation of the dioxo-Mo(V) anions [LMoO(2)(S(2)PR(2)-S)](-), which were slowly converted to the analogous oxothio-Mo(V) complexes [LMoOS(S(2)PR(2)-S)](-). Dioxygen reacted with [LMoOS(S(2)PPr(2))](-) to produce the oxothio-Mo(VI) complex LMoOS(S(2)PPr(2)-S). The (hydroxo)oxo-Mo(V) complexes LMoO(OH)(S(2)PR(2)-S) were formed upon reduction of LMoO(2)(S(2)PR(2)) with PPh(3) in wet (3-5 M H(2)O) tetrahydrofuran or upon ferrocenium oxidation of LMoO(S(2)PR(2)) in wet tetrahydrofuran. In dry solvents, LMoO(S(2)PR(2)) were oxidized to the corresponding cations, [LMoO(S(2)PR(2)-S,S')](+), which reacted with water to form LMoO(OH)(S(2)PR(2)). The Mo(V) complexes have been characterized by EPR spectroscopy.

Reaction of Mo IV O(S 2 PR 2 ) 2 with K{HB(Pr i pz) 3 } [HB(Pr i pz) 3 -) hydrotris(isopropylpyrazol-1-yl)borate] in refluxing toluene affords green L*Mo IV O(S 2 PR 2 -S,S′) complexes [L* ) HB(3-Pr i pz) 2 (5-Pr i pz) -) hydrobis(3-isopropylpyrazol-1-yl)(5-isopropylpyrazol-1-yl)borate; R ) Pr i , Ph], which are converted upon reaction with boron sulfide in dichloromethane to the yellow thio analogues L*Mo IV S(S 2 PR 2 -S,S′). Crystals of L*Mo IV O(S 2 PPr i 2 ) are monoclinic, space group P2 1 /n, with a ) 10.024 (2) Å, b ) 20.999(9) Å, c ) 15.368(5) Å, ) 100.57(2)°, V ) 3180(2) Å 3 , and Z ) 4. Crystals of L*Mo IV S(S 2 PPh 2 ) are monoclinic, space group P2 1 , with a ) 10.801(8) Å, b ) 13.100(5) Å, c ) 12.023(9) Å, ) 99.56(10)°, V ) 1678(2) Å 3 , and Z ) 2. The mononuclear, distortedoctahedral complexes are isostructural and are composed of a terminal chalcogenide ligand [ModO ) 1.671(3) Å, ModS ) 2.126(3) Å], a bidentate dithiophosphinate-S,S′ ligand, and a facial, tridentate L* ligand. In both cases the 5-isopropylpyrazole group is bound trans to the ModE group (E ) O, S). Oxygen atom transfer to L*Mo IV S(S 2 PR 2 ) and sulfur atom transfer to L*Mo IV O(S 2 PR 2 ) produce the oxo-thio-Mo(VI) complexes L*Mo VI -OS(S 2 PR 2 -S). NOESY experiments confirm that the 5-isopropylpyrazole group is trans to the ModO group in these chiral complexes. Ferrocenium oxidation of L*Mo IV E(S 2 PR 2 ) yields the corresponding oxo-and thioMo(V) complexes [L*Mo V E(S 2 PR 2 -S,S′)] + , respectively; the [L*Mo V O(S 2 PR 2 -S,S′)] + complexes react with water to produce (hydroxo)oxo-Mo(V) species, L*Mo V O(OH)(S 2 PR 2 -S), while the thio analogues are decomposed by water. The complex L′Mo V OCl 2 [L′ ) HB(3-Pr i pz) 3 -) hydrotris(3-isopropylpyrazol-1-yl)borate] was prepared by reacting L′Mo VI O 2 Cl with PPh 3 in dichloromethane; this complex does not react with sulfiding agents to produce the analogous thio complex. IntroductionThe simple replacement of a terminal oxo ligand by a terminal thio 2 ligand is a reaction plagued by practical limitations; the facile reduction of high-valent metal centers by sulfide and the formation of polynuclear µ-thio complexes usually conspire to prevent clean reactions. Nevertheless, this reaction has played a vital role in the generation of [Mo VI OS] 2+ , [Mo V -OS] + and [Mo V O(SH)] 2+ complexes, some of which are important models for the molybdenum centers formed during turnover of xanthine oxidase and xanthine dehydrogenase. [3][4][5] Most oxo-thio-Mo(VI) complexes have been produced by sulfidation of oxo complexes; examples include the oxothiomolybdates, [Mo VI O 4-n S n ] 2-, 6 the Wieghardt complexes Mo VI -OS(ONR 2 ) 2 , 7 and the organometallic species (η 5 -C 5 Me 5 )Mo VI -OS(CH 2 SiMe 3 ). 8 Generally, however, sulfidation of oxoMo(VI) complexes does not lead to clean replacement of an

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