The reaction between M(2)(OR)(6) and KOR in tetrahydrofuran (THF) in the 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-) for R = CH(2)(t)Bu. The anion, Mo-2(OCH(2)(t)Bu)(7)(-), has one bridging OR ligand whose oxygen atom is distinctly pyramidalized to avoid an unfavorable filled-filled Op pi with M-M pi orbital interaction. The Mo-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 even at -78 degrees C. Other M(2)(OR)(7)(-) anions show similar spectroscopic properties. The dianions is favored for M = W and R = CH(2)(t)Bu. The K+(crown) salts of M(+)(OCH(2)(t)Bu)(8)(2-) were structurally characterized (M = Mo and W) and shown to possess a M=M bonded unit centered with an O-8 "cube". The K+ ions coordinate four O atoms on opposite faces perpendicular to the M-M vector and are ligated by pyridine when crystals are own in its presence. The M-M distances 2.33(1) Angstrom (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, because of (i) their coordination to K+ and (ii) the increase in coordination 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 insoluble in non-coordinating hydrocarbons but dissolve upon the addition of 1 equiv of pyridine. The W-2(OR)(7)(-) anions are thermally labile yielding 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)Bu 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 independent of added KOR. The kinetic isotope effect k(H)/k(D) for the protio versus perdeuterio W-2(O(t)Bu)(7)(-) complex was 3.5(1) at 23 degrees C and 3.0(1) at 60.7 degrees C. From the decomposition of [W-2(OC(CH3)(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 retro-ene reaction is proposed.