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The photoreduction of uranyl solutions on illuminated TiO, suspensions and electrodes has been investigated. The photoreduction leads to the formation, on TiO, , of an uranium oxide having a stoichiometry close to U, O, .Adsorption of uranyl species in the dark was examined. It was found that the degree of adsorption depended on pH and on the formation of uranyl complexes in solution.Initial rates of photoreduction are given for the cases when propan-2-01, sodium acetate or sodium formate are used as scavengers of the photogenerated holes.With the use of microelectrode theory, it was possible to calculate the photoreduction rates from photoelectrochemical data obtained with TiO, electrodes. The reduction rates calculated in this way were from 3.5 to 4 times lower with the electrode than with the suspension, in agreement with the larger area available in the latter case.

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The photolysis of a benzene solution of [Tp(Me2)IrH(2)(COE)], 1 (Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate, COE = Z-cyclooctene), in the presence of P(OMe)(3), gives the stable novel complex [Tp(Me2)IrH(C(6)H(5))(P(OMe)(3))], 3a. The photochemical syntheses of [Tp(Me2)IrH(2)(P(OMe)(3))], from 1 and P(OMe)(3) in diethyl ether, and [Tp(Me2)IrH(2)(CH(2)=CHCOO(t)Bu)], from 1 in tert-butyl acrylate, are also reported. The above reactions and several experiments using C(6)D(6) and P(OCD(3))(3) show that, in all cases, the primary photoproduct is the 16-electron, five-coordinate iridium(III) intermediate {Tp(Me2)IrH(2)}, 6a, produced by loss of COE from 1. The above experiments also allow the postulation of a mechanistic pathway for the formation of 3a which involves the oxidative addition of an aromatic C-H bond by 6a. Furthemore, the photochemical reaction of 1 in the presence of P(OCD(3))(3) shows that, under the reaction conditions used, oxidative addition of C-H bonds of P(OMe)(3) and of coordinated Tp(Me2)-ligands, presumably, to the intermediates 6a and {Tp(Me2)IrH(C(6)H(5))}, also occurs. Thus, coordinatively unsaturated iridium(III) species readily activate C-H bonds.

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