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Ir(CO)2(acac)] in the pores of N a Y zeolite was treated in CO and converted into [Ir4(CO)12]. The zeoliteencaged [Ir4(C0) 121 was characterized by infrared and extended X-ray absorption fine structure spectroscopies, with the data indicating a n average Ir-Ir coordination number of 2.6 and a n average Ir-Ir distance of 2.69 A, in agreement, within the experimental error, with the published crystallographic data for solid [Ir4(C0)12]. Structurally simple zeolite-encaged iridium clusters were made by decarbonylation of the [Ir4(CO)12] a t 325 O C in flowing He followed by Hz. The decarbonylated clusters had an average Ir-Ir coordination number of 3.4 and a bond distance of 2.70 A, consistent with the inference that the tetrahedral framework structure of [Ir4(C0) 121 had been retained after decarbonylation; thus, the cluster is represented as tetrahedral Ir4. Infrared spectra showed that [Ir4(CO)12] was re-formed when the sample was treated in CO at 60 O C .

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Ir(CO)~(acac)] in the pores of NaX zeolite was treated in CO and converted into indium carbonyl clusters, which are suggested on the basis of infrared spectra and comparisons with solution chemistry to be [HIr4(CO), I]-, which was subsequently converted into clusters that are suggested to be [Ir6(C0)15I2-. The former clusters in the NaX zeolite were decarbonylated by treatment in He followed by H2 at 573 K and characterized by extended X-ray absorption fine structure spectroscopy. The Ir-Ir first-shell coordination number (3.0) indicates that the clusters were predominantly Ir4 tetrahedra. The clusters postulated to be [Ir6(C0)15]2-were reversibly fragmented and reformed in the cages.

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In attempts to prepare highly dispersed supported palladium catalysts stabilized by molybdenum, an organometallic precursor with Pd-Mo bonds, [ P~~M O~( C P ) Z ( C O >~( P P~~)~~, was adsorbed on MgO. The precursor Was adsorbed intact, as shown by infrared spectroscopy. For comparison, other samples were prepared from an organopalladium precursor, [PdClz(PhCN)2], and from a mixture of [PdC12(PhCN)2] + [Mo(C0)6].Each supported sample was treated in H2 at various temperatures to form metallic palladium. The palladium dispersions were characterized by chemisorption of H2, CO, and 0 2 ; transmission electron microscopy; temperature-programmed desorption of adsorbed CO; and extended X-ray absorption fine structure (EXAFS) spectroscopy, both at the Pd K edge and the Mo K edge. The data show that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form, with the supported clusters being smaller than about 10 A in average diameter. These clusters have a low capacity for chemisorption of hydrogen and of CO. They are stabilized by the oxophilic molybdenum, which exists preferentially at the interface between the palladium clusters and the metal oxide support. The sample prepared from the two monometallic precursors was characterized by larger palladium particles and by weaker PdMo interactions. The results suggest that the Pd-Mo interactions in the bimetallic precursor were the cause for the high dispersion of palladium in the reduced catalyst.