Mononuclear rhenium carbonyls on MgO : characterization of the metal-support interface by extended X-ray absorption fine structure spectroscopy

Structures formed on the surface of fully dehydroxylated MgO by adsorption of [HRe(CO)[sub 5]] were characterized by extended x-ray absorption fine structure (EXAFS) spectroscopy. The rhenium carbonyl initially was molecularly adsorbed, interacting with the MgO surface as the proton was partially removed by a basic oxygen of the surface; the average Re-C and Re-O* coordination numbers (where O* refers to the carbonyl oxygen) are about 5. There is no EXAFS evidence for interactions of carbonyl oxygen with the surface. The average Re-C and Re-O* distances are 1.94 and 3.13 [angstrom], respectively. When the adsorbed [HRe(CO)[sub 6]] was heated to 80C in hydrogen, it lost two CO ligands per Re and formed a sub-carbonyl formulated as [Re(CO)[sub 3][OMg][HOMg][sub 2]], where the brackets refer to groups terminating the bulk MgO. The average Re-C, Re-O*, and Re-O distances are 1.91, 3.12, and 2.13 [angstrom], respectively. The Re-O distance is equal, within [plus minus] 0.1 [angstrom], to the metal-oxygen distances in the only other well-characterized metal sub-carbonyls on metal oxides and to the metal-oxygen distances in numerous organometallic complexes with oxygen-containing ligands.