A Longoni-Chini cluster, [Ptl5(C0),l2-, was prepared on MgO powder by a surfacemediated synthesis from Na2PtCb in the presence of CO. The formation of [Pt15(CO)30]2-and its decarbonylation at 120 "C under vacuum were characterized by infrared and X-ray absorption spectroscopies. The decarbonylated cluster had an average Pt-Pt bond distance of 2.76A and an average Pt-Pt first-shell coordination number of 3.7, indicating that the trigonal prismatic framework structure of the Longoni-Chini cluster was retained after decarbonylation; thus the decarbonylated cluster is represented as PtIS with a trigonal prismatic structure. When this sample was recarbonylated, the Longoni-Chini cluster was not re-formed. The average Pt-Pt bond distance increased to 2.78 A and the Pt-Pt first-shell coordination number increased to 6.0, indicating that a CO-induced morphology change gave approximately hemispherical Pt clusters with adsorbed CO. IntroductionStructurally simple supported metals have been prepared from carbonyl clusters of Os,] Ir,2 and a few other metals, but not yet from those of Pt. Pt is the most important catalytic metal, being applied for automobile exhaust conversion, naphtha reforming, and many other reactions. Preparation of structurally simple supported catalysts from Pt carbonyl clusters is difficult (1) because Pt carbonyls are highly air sensitive and difficult to synthesize in high yields on support surfaces and (2) Here we report the surface-mediated synthesis of [Ptls(CO),l2-on MgO, its decarbonylation (without loss of nuclearity), and its recarbonylation (with changes in nuclearity). The surface structures have been characterized by infrared and X-ray absorption spectroscopies.Characterization of Surface and Extracted Species by Infrared and Ultraviolet-Visible Spectroscopy. The solid formed by deposition of Na2PtC1, on MgO followed by treatment in C O was yellow-green. The surface species were characterized by infrared spectroscopy and by extraction into solution. The adsorbed species could not be extracted with neutral solvents such as THF, but extraction with [PPN] [Cl] in THF was efficient, consistent with the Occurrence of cation metathesis, which implies the presence of ionic species on the MgO surface. (Table which is therefore identified as the organometallic species in the extract solution. The yield of [Pt15(CO)30]2-in the one-step extraction, estimated from the absorbance at 706 nm, was about 73%.The uc0 infrared spectrum of the surface species ( on MgO did not change in either intensity or position as the sample was heated under vacuum (0.01-0.001 Torr) to 65 OC ( Figure 3A). The bridging C O band disappeared, and the intensity of the terminal CO absorption decreased to about one-third of the original value after 10 min at 75 OC under vacuum ( Figure 3B). After 30 min at 75 OC, the intensity of the terminal CO band had decreased to about 10% of its original value ( Figure 3C). The decarbonylation was complete after 30 min at 100 O C under vacuum ( Figure 3D).After the sample h...

A metal carbonyl cluster with a tetrahedral metal frame, [Hk4(CO)11]-, was prepared on the surface of partially hydroxylated MgO powder by adsorption of [Ir4(CO)12]. A supported catalyst was prepared by decarbonylation of the supported [Hzr4(CO)11]-by treatment in He followed by Hz at 300 "C. The surface structures were characterized by inf'rared and X-ray absorption spectroscopies. The decarbonylated cluster had an average Ir-Ir coordination number of 3.4, consistent with the hypothesis that the tetrahedral metal frame was largely retained after decarbonylation. The sample incorporating the decarbonylated clusters was catalytically active for propane hydrogenolysis at 1 atm and 200 "C. The catalyst that had been used for 1 day in a flow reactor was characterized by an Ir-Ir coordination number of 3.3. The lack of any significant change in the average cluster size indicates the stability of the supported catalyst, which is modeled as tetrahedral Ir4 clusters. The rate of the propane hydrogenolysis at 200 "C and atmospheric pressure with a H2/propane molar ratio of 2.5 was 0.02 molecule of CHq produced(Ir4 cluster s).

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