A valence bond perspective on the molecular shapes of simple metal alkyls and hydrides

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.