A Family of Rhenium Subcarbonyls on MgO: Structural Characterization by Extended X-ray Absorption Fine Structure Spectroscopy

Abstract

Stable rhenium tricarbonyls bonded to the surface of MgO powder, prepared from a variety of precursors including [Re[sub 2](CO)[sub 10]], [DRe(CO)[sub 8]], and [Re[sub 4](CO)[sub 12](OH)[sub 4]], have been characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. The conversion of the initially adsorbed species into the rhenium subcarbonyl has been carried out by a variety of routes at relatively mild temperatures, including treatment in H[sub 2], He, O[sub 2], or under vacuum. The EXAFS and infrared data indicate a C[sub 3v] symmetry of the surface-bound rhenium subcarbonyls. Each Re atom is bonded to three surface oxygen atoms, and the complex is formulated as [Re(CO)[sub 3][l brace]OMg[r brace][sub x][l brace]HOMg[r brace][sub 3[minus]x]] (where the braces denote groups terminating the MgO bulk). The surface species are precise analogues of molecular rhenium carbonyls, as indicated by both the coordination numbers and bonding distances for all the members of the family. The indications of this symmetry demonstrate that the rhenium subcarbonyls on the surface of MgO powder are bound to faces other than the (100) face and/or at surface defects. These rhenium subcarbonyls are among the best characterized examples of adsorbates on metal oxide powders. 22 refs., 4 figs., 6 tabs.