DIMETAL HEPTAALKOXIDE AND OCTAALKOXIDE ANIONS OF MOLYBDENUM AND TUNGSTEN, M(2)(OR)(7)(-) AND M(2)(OR)(8)(2-) (M-M) - PREPARATION, STRUCTURES, OXIDATION, AND A STUDY OF THE THERMAL-DECOMPOSITION OF W-2(OR)(7)(-) TO GIVE W-2(H)(O)(OR)(6)(-) WHERE R=(T)BU AND PR-I

The reaction between M(2)(OR)(6) and KOR in tetrahydrofuran (THF) in t he presence of 18-crown-6 leads to the reversible formation of K(crown )(+)M(2)(OR)(7)(-) (R = (t)Bu, Pr-i) along with 2K(+)M(2)(OR)(8)(2-) f or R = CH(2)(t)Bu. The anion, Mo-2(OCH(2)(t)Bu)(7)(-), has one bridgin g OR ligand whose oxygen atom is distinctly pyramidalized to avoid an unfavorable filled-filled Op pi with M-M pi orbital interaction. The M o-Mo distance, 2.218(1) Angstrom (av), is similar to that in Mo-2(OCH( 2)(t)Bu)(6) [Chisholm, Cotton Murillo, Reichert Inorg. Chern. 1977, 16 , 1801]. In solution the molecule is fluxional on the NMR time scale e ven at -78 degrees C. Other M(2)(OR)(7)(-) anions show similar spectro scopic properties. The dianions is favored for M = W and R = CH(2)(t)B u. The K+(crown) salts of M(+)(OCH(2)(t)Bu)(8)(2-) were structurally c haracterized (M = Mo and W) and shown to possess a M=M bonded unit cen tered with an O-8 ''cube''. The K+ ions coordinate four O atoms on opp osite faces perpendicular to the M-M vector and are ligated by pyridin e when crystals are own in its presence. The M-M distances 2.33(1) Ang strom (av) (M = W) and 2.256(2) Angstrom (M = Mo) are well within the range seen for (M=M)(6+)-containing compounds while the M-O distances are ca. 2.0 Angstrom, notably longer than in M(2)(OR)(6) compounds, be cause of (i) their coordination to K+ and (ii) the increase in coordin ation number at the metal. Oxidations of the M(2)(OCH(2)(t)Bu)(8)(2-) anions with Ph(3)PBr(2) cleanly yield M(2)(OCH(2)(t)Bu)(8) along with KBr and PPh(3). M(2)(OCH(2)(t)Bu)(8) (M = Mo, W) are essentially insol uble in non-coordinating hydrocarbons but dissolve upon the addition o f 1 equiv of pyridine. The W-2(OR)(7)(-) anions are thermally labile y ielding isobutylene (R = (t)Bu) or propene (R = Pr-i) and W-2(mu-H)(mu -O)(OR)(6)(-). In pyridine-d(5) the activation parameters are Delta H dagger = 23.1(5) kcaI/mol and Delta S dagger = -3.3(5) eu for R = (t)B u and Delta H dagger = 23.8(8) kcal/mol and Delta S dagger = -4(2) eu for R =Pr-i. The reactions were first order in W-2(OR)(7)(-) and indep endent of added KOR. The kinetic isotope effect k(H)/k(D) for the prot io versus perdeuterio W-2(O(t)Bu)(7)(-) complex was 3.5(1) at 23 degre es C and 3.0(1) at 60.7 degrees C. From the decomposition of [W-2(OC(C H3)(CD3)(2))(7)]- and the ratio of the liberated CH2=C(CD3)(2) to CD2= C(CH3)(CD3) the absolute value of k(H)/k(D) = 3.5(4) at 23 degrees C. The hydride ligand is chemically inert to olefins and to exchange with the deuterium of (t)BuOD. Plausible pathways for the formation of the oxo hydride are discussed and a cyclic transition state akin to a ret ro-ene reaction is proposed.