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Stable rhenium tricarbonyls bonded to the surface of MgO powder, prepared from a variety of precursors including [Rez(CO)lo], [DRe(C0)61, and [Re4(C0)12(OH)41, 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 H2, He, 0 2 , or under vacuum. The EXAFS and infrared data indicate a C3u 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)3{ OMg},{HOMg}~-,I (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.

The decarbonylation of [Rez(CO)lo] on ultrathin MgO( 11 1) films (-50 A) grown on a Mo ( 110) substrate was investigated with infrared reflection-absorption spectroscopy and temperature-programmed desorption/ decomposition. When [Re2(CO)lo] was adsorbed at 320 K, it reacted with the MgO(ll1) surface, forming a rhenium subcarbonyl, [Re(CO)4{OMg}12, where {OMg] represents oxygen on the MgO(ll1) surface. Upon heating to temperatures >400 K, [Re(C0)4{OMg}] 2 was decarbonylated, forming [Re(CO)3{OMgJ3]. Further heating to temperatures >600 K induced the formation of rhenium metal clusters. When it was adsorbed at 110 K, [Re2(CO)lo] condensed on MgO( 1 I l), with a fraction of the first layer decomposing to form [Re-(C0)4{0Mg}] 2. In temperature-programmed desorption experiments, [Re2(CO)lo] multilayers desorbed at 295K. An unidentified rhenium carbonyl species, probably rhenium pentacarbonyl, desorbed at 350 K. The C O stretching frequencies of the rhenium subcarbonyls were dependent on the coadsorption of water. The infrared spectra show that the symmetry of the surface-bound rhenium tricarbonyls is C3". The results are among the first characterizing the adsorption of metal complexes on well-defined metal oxide surfaces, and they demonstrate the value of structurally characterized supports for elucidation of the nature of the metal-support interface.

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