Transition metal and excited state electronic structure calculations employing a localized bonding perspective

Modern electronic structure calculations are increasingly employed to understand unique reactivity patterns and electronic excitation energies involving transition metals. From a quantum mechanical wavefunction, Natural Bond Orbital (NBO) analysis extracts the "best possible" set of one- and two-center orbitals. Insights gained from a localized perspective have assisted in guiding quantum calculations that lead to a concrete and quantitative model of some transition metal reactions and electronic excitations.; Two chapters probe the electronic rearrangements underlying the reaction of dioxygen with the sixteen-electron palladium(0) compound (bathocuproine)Pd(dba) in dichloromethane to form a sixteen-electron peroxopalladium(II) product, (bathocuproine)Pd(dioxygen). Using spin-unrestricted DFT, the reactions of dioxygen with the related species (ethylenediamine)Pd and (ethylenediamine)Pd(ethylene) are modeled. Spin-crossover is facilitated by stepwise electron transfer. Charge donation from a Pd axial lone pair into a ligand acceptor orbital, and transient isomerization through a pseudo-octahedral structure, connect the mechanism with that of olefin ligand exchange.; The next chapter reports a systematic density functional (DFT) study of metal-ligand bond enthalpies for saturated transition metal complexes that encompasses the entire d-block of the periodic table and a wide assortment of ligands. Using NBO, Landis and coworkers developed a simple duodectet rule describing the valency of transition metals, to extend the octet rule for main group elements. Accordingly, closed-shell transition metal molecules are formed with two-center, two-electron bonds to the ligand in question and the maximum number of auxiliary hydrides, such that the metal electron count does not exceed twelve. Periodic trends in bond enthalpies are explored within this consistent data set.; The last chapter introduces a new method for constructing approximate wavefunctions with specified Lewis-like electronic configurations. Example calculations and progress toward a "black box" implementation are reported. Excited electronic configurations differ from the ground state Lewis structure in the occupancy of one or more localized orbitals. Using highly transferable NBOs, self-consistent field (SCF) calculations are guided to converge upon unusual wavefunctions such as electronic excited states.