Spin-orbital electronegativity, the Xα method, and reactivity at transition-metal interfaces

It is shown that the concept of electronegativity, originally viewed as a virtually constant characteristic of an atom, can be generalized to the individual molecular orbitals of aggregates of atoms, utilizing the self-consistent field X-alpha (SCF-Xα) density-functional representation of molecular-orbital theory in conjunction with the definition of orbital electronegativity proposed by Hinze et al. This generalization allows for the dependence of electronegativity on the detailed electronic structure of a group of atoms as a function of its composition, geometry, and local chemical environment. In transition metals and transition-metal coordination complexes, where local magnetic spin polarization of electrons is important, the concept of orbital electronegativity can be further generalized to the individual spin-orbitals. By viewing a transition-metal surface, cluster, or coordination complex as providing orbital or spin-orbital pathways for electrons to effectively flow between reactants, where such flow directly between reactants in the gas phase is forbidden by orbital-symmetry restrictions or unfavorable electronegativity differences, orbital or spin-orbital electronegativity can be used in conjunction with SCF-Xα calculations for representative clusters and complexes as an approximate index of heterogeneous or homogeneous reactivity. Recent applications of this concept to a number of problems associated with reactivity at transition-metal interfaces are reviewed, including: (1) the dissociation and reactivity of hydrogen at low-coordination transition-metal sites, (2) the interaction of atomic hydrogen with transition-metal interfaces, and (3) the surface reactivity of iron.