A FAMILY OF RHENIUM SUBCARBONYLS ON MGO - STRUCTURAL CHARACTERIZATIONBY EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY

Stable rhenium tricarbonyls bonded to the surface of MgO powder, prepa red from a variety of precursors including [Re-2(CO)(10)], [DRe(CO)(5) ], and [Re-4(CO)(12)(OH)(4)], have been characterized by extended X-ra y absorption fine structure (EXAFS) spectroscopy. The conversion of th e initially adsorbed species into the rhenium subcarbonyl has been car ried out by a variety of routes at relatively mild temperatures, inclu ding treatment in H-2, He, O-2, or under vacuum. The EXAFS and infrare d data indicate a C-3v symmetry of the surface-bound rhenium subcarbon yls. Each Re atom is bonded to three surface oxygen atoms, and the com plex is formulated as [Re(CO)(3){OMg}(x){HOMg}(3-x)] (where the braces denote groups terminating the MgO bulk). The surface species are prec ise 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 th an the (100) face and/or at surface defects. These rhenium subcarbonyl s are among the best characterized examples of adsorbates on metal oxi de powders.