These results could well suggest that all future ab initio calculations on substituted carboranes be conducted at least to the ( 13) Clark, T. Computational Chemistry; Wiley-Interscience: New York, 1985. (14) (a) Oh, B.; Onak, T. Inorg. Chem. 1982,21,3150-3154. (b) Onak, T.; Fung, A. P.; Siwanpinyoyos, G.; Leach, J. B. Inorg. Chem. 1979, 18, ( I 5) Beltram, G. A.; Jasperse, C.; Cavanaugh, M. A.; Fehlner, T. P. Inorg. Chem., preceding paper in this issue. (16) (a) Takimoto, C.; Siwapinyoyos, G.; Fuller, K.; Fung, A. P.; Liauw, L.; Jarvis, W.; Millhauser, G.; Onak, T.

bond angles, bond distances, torsional angles, positional parameters of H-atoms, general temperature factors, and structure factor amplitudes for 1 and 6(110 pages). Ordering information is given on any current masthead page.

The photoinduced oxidation of tetra(Nmethyl-4-pyridyl)porphyrinmanganese(III) has been achieved in homogeneous solution. The manganese porphyrin was used as an electron donor in a three-component system with tris-(2,2'-bipyridine)ruthenium(II) as the photosensitizer and chloropentaamminecobalt(III) as the electron acceptor. The photooxidized manganese porphyrin is unstable in aqueous solution, reverting to the starting manganese(HI) porphyrin. The oxidation of manganese(HI) porphyrin and the subsequent reduction of the oxidized porphyrin can be cycled repeatedly.Much attention has been given to the study of photochemical generation of hydrogen and oxygen from water as a means of collecting and storing solar energy (1-3). A practical device for such solar energy conversion should be capable of generating hydrogen and oxygen simultaneously. While considerable progress has been made in achieving the photoinduced hydrogen evolution from water (ref. 3, pp. 99-122), only few advances have been made in the photogeneration of oxygen (4-6). The basic reason for the lack of progress in oxygen evolution is that the oxidation of water to oxygen involves four electrons. Nevertheless, photochemical generation of oxygen from water has been reported with the use of heterogeneous catalysts (4-6). In these experiments, tris(2,2'-bipyridine)ruthenium(II), Ru(bpy)2+, was used as a photosensitizer and chloropentaamminecobalt(III), CoCl(NH3) +, as an electron acceptor. Our work is directed at the development of homogeneous catalysts for the evolution of oxygen.In photosynthesis, manganese complexes are involved in the oxygen-evolution process; the exact nature of these complexes is not known (7). Manganese complexes exhibit a rich variety of oxidation states, making them promising oxidation catalysts (8), and, in addition, it has been reported that manganese-porphyrin complexes catalyze the oxidation of olefinic hydrocarbons (9). Therefore, it is of interest to see whether the porphyrin complexes can catalyze the oxidation of water. Since manganese-porphyrin complexes with manganese in higher oxidation states can form dimeric species by bridging two oxygen atoms obtained from water molecules, these complexes might be able to liberate oxygen from water (10). A first and necessary step in the chemical solar energystoring system is the driving of a thermodynamically unfavored reaction by absorption of photons. In the present paper we report the photochemical oxidation of a watersoluble manganese(III) porphyrin, tetra(N-methyl-4-pyridyl)-porphyrinmanganese(III), Mn(III)Por+.As manganese(III)-porphyrin complexes are short-lived in their excited states, use of these complexes as photosensitizers to achieve their oxidation is limited (11,12). The approach taken in the present investigation is to use the manganese porphyrin as an electron donor in a threecomponent system consisting of a photosensitizer, an electron acceptor, and an electron donor. A schematic arrangement of this system is shown in Fig. 1. In this study, Ru(bpy)32 i...

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