A valence bond description of transition metal bonding.

摘要

A Valence Bond approach to the description of the molecular shapes and bonding of transition metal compounds is presented. A simple set of rules prescribes the generation of Lewis structures for transition metal compounds. As in main group compounds, the Lewis structure picture of localized bonding gives insight into the forces defining molecular shape. Transition metals are shown to use only the s and d orbitals in the compounds studied, restricting the normal valence space to twelve electrons and resulting in shapes of localized orbitals different from those seen for spn hybridized main group compounds.;The localized description of transition metal bonding is ideally suited for incorporation into a Molecular Mechanics algorithm. The Valbond Molecular Mechanics program has been adapted to incorporate localized bonding descriptions of transition metals. Transition metal hydrides and alkyls, many of which have unusual, low symmetry structures, can be readily rationalized by Valence Bond theory and modeled using Valbond. The success of the Molecular Mechanics algorithm lends further credence to the underlying Valence Bond theory model.;Multiple bonding between a ligand and a transition metal also has been examined, both theoretically and by application to the Valbond program. Bond orders for transition metal-ligand bonds are predicted by drawing Lewis structures. These predicted bond orders are revealed to be accurate by examination of calculated and experimental geometries of transition metal compounds. Notably, localized double bonds are shown to have an asymmetric effect on other ligands bonded to the metal, resulting in unusual low-symmetry shapes. These unusual shapes can be understood with Valence Bond theory and calculated with Valbond. Although Valbond is currently most accurate in modeling structures which can be described by a single resonance structure, future algorithm development might enable it to model the entire periodic table.