Bare transition metal atoms in the gas phase: reactions of M, M+, and M2+ with hydrocarbons

Abstract

This Account focuses on gas-phase measurements and high quality ab initio calculations that are beginning to explain how metal atom electronic structure determines chemical reactivity. The authors have an enormous body of basic chemical reactivity data for M[sup +] and a growing body of data for M[sup 2+] and neutral M. Sophisticated experiments can control the kinetic energy and the electronic state of M[sup +] reactants. One can study the reactivity of Fe[sup +] in the 3d[sup 6]4s, high-spin ground state, the 3d[sup 7], high-spin first excited state, or the 3d[sup 6]4s, low-spin second excited state. The authors have learned to follow the elimination of H[sub 2] and C[sub 2]H[sub 6] from bimolecular Ni[sup +](n-butane) complexes in real time, on a 50-ns time scale. In M[sup +] reactions, the authors can control the kinetic energy and the electronic state of the reactants. A key advantage in interpreting these results is that one understands the electronic structure of the bare metal atom reactants very well. Solution-phase chemists might well question the relevance of atomic species with genuine 1+ or 2+ charges and no ligands or solvent to the [open quotes]real world[close quotes] of organometallic chemistry. Yet connections surely exist, as witnessedmore » by the fact that Rh and Ir atoms are unusually reactive in all phases. Theoretical chemists are beginning to provide a conceptual framework that will unify seemingly diverse branches of experimental chemistry. Of necessity, the ab initio quantum chemist works on model transition metal systems, draws experimental evidence from all available sources, and tries to abstract from the calculations what is robust and common to all phases. Gas-phase metal atoms are idealized model systems well matched to the capabilities of modern theory. Many new conceptual insights in the next 10 years will come from careful analysis of ab initio wave functions informed by incisive gas-phase experiments. 30 refs., 5 figs., 1 tab.« less