REACTIONS OF TETRAIRIDIUM CLUSTERS ON THE SURFACE OF MGO - CHARACTERIZATION BY INFRARED-SPECTROSCOPY AND CHEMISORPTION MEASUREMENTS

Infrared spectroscopy was used to characterize the preparation of [HIr 4(CO)(11)](-) from [Ir-4(CO)(12)] on the surface of MgO powder at room temperature and the conversion of the supported [HIr4(CO)(11)](-) int o [Ir-6(CO)(15)](2-) by treatment in CO at 1 atm and 200 degrees C. Du ring the preparation of [HIr4(CO)11](-), OH groups on the MgO were par tially consumed, and surface carbonates formed; the surface reaction i s analogous to that by which [Ir-4-(CO)(12)] is converted into [HIr4(C O)11](-) in basic solutions. The decarbonylated cluster has been chara cterized by extended X-ray absorption fine structure spectroscopy (as reported elsewhere) and modeled as Ir-4. When the decarbonylated clust ers were recarbonylated, the infrared bands characteristic of MgO-supp orted [HIr4(CO)(11)](-) did not reappear; rather, new supported iridiu m carbonyls formed. Chemisorption of H-2, CO, and O-2 On the decarbony lated clusters was measured at room temperature. More O-2 was chemisor bed on the MgO-supported Ir-4 than on metallic iridium particles, wher eas the uptake of H-2 and of CO on the iridium clusters was less than that observed for metallic iridium particles. All the results imply th at the decarbonylated clusters should be considered to be quasi-molecu lar and not metallic.