Recently we reported a qualitative, valence bond derived model for describing the shapes of transition metal complexes, with a focus on metal hydrides and alkyls. This model, based on the concepts of hybridization and resonance, rationalizes the unusual and varied shapes of hydride and alkyl complexes with transition metals. This paper demonstrates the quantitative incorporation of these valence bond concepts into molecular mechanics algorithms. The resulting force field method (HV-VB) accurately describes the structures of alkyls and hydride complexes of the transition metals. For a wide variety of crystallographically characterized molecules, the HV-VB computations faithfully reproduce the observed structures.

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Many chemists use qualitative valence bond concepts to rationalize molecular structures and properties, particularly for main group elements. Extension of Pauling's valence bond concepts to transition metal compounds dominated by covalent bonding leads to simple prescriptions for determining bond hybridizations and molecular shapes. As a result, transition metal structures can be discussed in the familiar terminology of Lewis structures, lone pairs, hybrid orbitals, hypervalence, and resonance. A primary feature of these prescriptions is the relative impotence of valence p-orbitals in the formation of covalent bonds at transition metals:  sd n hybridization dominates. This feature is consistent with detailed analyses of high level quantum mechanical computations. Unlike Pauling's original treatments of hypervalency, rationalization of empirical structures and high level electronic structure computational results requires consideration of multiple resonance structures. Valence bond theory constitutes a compact and powerful model that accurately explains the often unexpected structures observed for simple metal alkyls and hydrides.

The method allows measurement of the total nitric oxide produced in in vitro experimental systems. Oxford Biomedical Research. Circle 144. U Cell Proliferation AssayA new assay allows researchers to simultaneously measure cell proliferation and study phenotypically distinct subsets of a mixed-5918 EVERGREEN/ ST. LOUIS, MISSOURI 63134 U.S.A.