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carbonylation. Compound 9 contains a square pyramid of five osmium atoms with an Os(CO)3 capping group. It seems reasonable to expect that the analogous Ru6-(CO)17(m4-S) could be prepared by the decarbonylation of 3; however, this has not yet been achieved.The addition of yet another ruthenium carbonyl fragment results in the formation of Ru7(CO)19(m-CO)2(m4-S) (4). This product was made independently by the addition of a mononuclear fragment to 3. The structure of 4 shows that it contains two Ru(CO)4 groups which bridge diametrically opposed basal edges of the square pyramid and was formed by the replacement of a second bridging CO ligand in 3 with a Ru(CO)4 group. The osmium homologue of 4 has not yet been reported. The closest related osmium compound is Os7(CO)19(g4-S) which contains a sulfurbridged square pyramid of five osmium atoms that is fused to a trigonal-bipyramidal cluster of five osmium atoms through a triangular face.13 Compounds 2, 3, and 4 can be degraded by treatment with CO at 98 °C. Curiously, small amounts of 4 were formed in the degradation of 3 and small amounts of 3 and 4 were obtained in the degradation of 2. The degradation reactions will lead to the formation of mononuclear ruthenium fragments which could be added to unreacted clusters. It is believed that under suitable conditions cluster enlargement and degradation will be ongoing and competing reactions in solutions that contain the appropriate species. These transformations are summarized in Scheme I.Acknowledgment. The research was supported by the National Science Foundation under Grant No. CHE-8612862.

The energies of the lowest singlet (S,) and triplet ( T I ) states of 28 molecules have been calculated by the " half-electron'' (MNDO-HE) and spin-unrestricted (UMNDO) versions of MNDO. While most of the calculated values are too negative, because of overestimation of the correlation energy in MNDO-HE and UMNDO, the errors are systematic and depend in an understandable way on the nature of the molecular orbitals (MOS) involved. When appropriate corrections are applied, the calculated energies agree with experiment almost as well as they do for ground states. This justifies the use of MNDO-HE or UMNDO for studies of excited state processes.

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The dielectric constant in Coulomb's Law, D, can quantify an empirical reduction of force. It can also quantify a reduction of electrostatic field as seen in classical electrostatic theory where the induced charge layer is assumed to be infinitely thin. The two approaches exemplify two traditions that have been used in parallel for decades. They produce Potential Energy Functions (PEFs) that differ by a factor of the permittivity, εr. The classical electrostatic theory result can be incorporated into force field models with an effective dielectric function, Deff, which spans the induced charge layer and accommodates both traditions. The Deff function increases the magnitude of local terms as compared with cumulative long distance terms. It is shown that the Deff function reduces distance dependence of the radial PEF within the induced charge layer and improves computational stability for some systems including substrate in dilute salt solution. End use applications include pharmaceutical development (e.g. protein calculations with docking), materials development, solvation energy calculations and QM/MM calculations.