LIGAND EFFECTS IN SUPPORTED METAL-CARBONYLS - X-RAY ABSORPTION-SPECTROSCOPY OF RHENIUM SUBCARBONYLS ON MAGNESIUM-OXIDE

[HRe(CO)5] reacted with the surfaces of MgO powders, one being partially dehydroxylated (about 55%) and the other almost fully dehydroxylated (about 93%). The initial surface species were molecularly adsorbed [HRe(CO)5], which, upon heating to 80-degrees-C in H-2 or under vacuum, gave rhenium subcarbonyls with three CO ligands and three oxygen-containing ligands provided by the MgO surface. Infrared spectra are consistent with C3v symmetry in both structures, with bands at 2011 (vs), 1895 (vs), and 1862 (sh) cm-1 for rhenium subcarbonyl on the partially dehydroxylated MgO and at 2017 (vs), 1908 (vs), and 1867 (sh) cm-1 for the rhenium subcarbonyl on the almost fully dehydroxylated MgO. The average bond distances were determined by extended X-ray absorption fine structure spectroscopy. On the partially dehydroxylated MgO, the Re-C, Re-O* (O* is a carbonyl oxygen), and Re-O(s) (O(s) is a surface oxygen) distances were 1.87, 3.11, and 2.13 angstrom, respectively, and on the almost fully dehydroxylated MgO, the Re-C, Re-O*, and Re-O(s) distances were 1.91, 3.12, and 2.13 angstrom, respectively. These distances indicate chemical bonding of the rhenium carbonyl to oxygens of the MgO surface. They also demonstrate greater electron donation (backbonding) from the Re to the CO on the partially dehydroxylated MgO than from the Re to the CO on the almost fully dehydroxylated MgO surface, suggesting electron transfer from the MgO to the Re. The MgO surface is thus modeled as a rigid multidentate electron donor analogous to a molecular ligand. The X-ray absorption near edge data confirm a higher electron density on the Re atoms bonded to the oxygens of partially dehydroxylated MgO than on Re atoms bonded to almost fully dehydroxylated MgO.