Disparate product distributions observed in Mo_(3-x)W _xO_y^- (x=0-3; Y=3-9) reactions with D ₂O and CO₂

Results of gas phase reactivity studies on group six transition metal suboxide clusters, Mo₃O_y^-, Mo ₂WO_y^-, MoW₂O_y ^-, and W₃O_y^- (Mo _(3-x)W_xO_y^-, x=0-3; y=ca. 3-9) with both D₂O and CO₂ are reported. Sequential oxidation for the more reduced species, Mo_(3-x)W_xO_y ^- + D₂O/ CO₂ → Mo_(3-x)W _xO_y+1^- + D₂/CO, and dissociative addition for certain species, Mo_(3-x)W_xO_y ^- + D₂O/ CO₂ → Mo_(3-x)W _xO_y+1D₂^-/Mo_(3-x)W _xO_y+1CO-, is evident in the product distributions observed in mass spectrometric measurements. Reactions with D₂O proceed at a rate that is on the order of 10² higher than for CO₂. The pattern of reaction products reveals composition-dependent chemical properties of these group six unary and binary clusters. At the core of this variation is the difference in Mo-O and W-O bond energies, the latter of which is significantly higher. This results in a larger thermodynamic drive to higher oxidation states in clusters with more tungsten atoms. However, addition products for more oxidized W-rich clusters are not observed, while they are observed for the more Mo-rich clusters. This is attributed to the following: In the higher oxides (e.g., y=8), addition reactions require distortion of local metal-oxygen bonding, and will necessarily have higher activation barriers for W-O bonds, since the vibrational potentials will be narrower. The binary (x=1,2) clusters generally show sequential oxidation to higher values of y. This again is attributed to higher W-O bond energy, the result being that stable binary structures have W atoms in higher oxidation states, and Mo centers both in more reduced states and sterically unhindered. The reduced Mo center provides a locus of higher reactivity. An unusual result that is not readily explained is the chemically inert behavior of Mo₃O₆^-.