On the energetics of the formation of EHM from MHM interactions on transition metal clusters

Abstract

Experimental studies from several laboratories have demonstrated that, when a main group—transition metal atom cluster contains endo-cluster hydrogens, equilibria between tautomers which differ in the placement of one or more endo-hydrogens between MHM and EHM (E = main group atom, M = transition metal) bridging sites can exist. In this study we show that the preference for EHM versus MHM sites can be simply described in terms of the difference in electronegativities of the E and M atoms. This model allows a correlation of observed site preferences with calculated enthalpy change and allows predictions of site preference in presently unknown compounds. Further, as the model is independent of exo-cluster structure, it can be used to explain some aspects of the relative reactivity of main group atoms when bound to transition metal atoms at the surface or grain boundaries of a bulk metal.