Metal-oxygen bonds, bond energies

Metal-Oxygen Bonds Electronic Energy Levels and Chemical Bonding. 

Fig. 8. Energies calculated with a polarizable continuum model, differences of the sums of all metal-oxygen bond lengths, AS(M-O), and energy profiles for water exchange on rhodium(III) and ruthenium(II) hexaaqua ions.

Unfortunately, the absolute value of the metal-oxygen bond energy does not unambiguously define the level of selectivity in the partial oxidation process. In addition, the specific interactions between the substance being oxidized and the catalyst surface and the character of the metal-oxygen bond are important parameters as well. 

General reviews of the use of oxide semiconductors for the photoelectrolysis of water are contained in Refs. 62 and 65 (see Table 1). Eleven binary and ternary oxides were examined in Ref. 62. Linear correlations were presented between the flat band potential, Vtb of these oxides and their band gap energy (Eg) and between Vm and the heat of formation of the oxide per metal atom per metal-oxygen bond. Aligning all the oxide energy levels on a common scale, these authors noted62 that the position of the conduction band varies much more than those of valence bands - a trend expected from the cationic (d-band) character of the conduction band in the oxide while the valence band is mainly of 0(2p) character. The latter should be relatively independent of the oxide parentage in terms of the metal. 

Heats of Activation. Figure 4 shows a plot of AH vs. AH° for Reactions 7-17 in which an actinide metal-oxygen bond is formed. These data scatter considerably more than the corresponding free energy data but do show a distinct dependence on both AH° and on the number of... 

For Reactions 20-25 the heats are similar to the free energies in that, except for Reaction 23, AH is correlated with AH° the average AH is 16.1 kcal./mole for one hydrogen ion released. The heats of activation for the reactions in which metal-oxygen bonds are neither formed or broken (1-6) do not appear to be correlated with AH°. 

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