Platinum-containing metal clusters

F, 1 NMR studies on the dynamic behaviour in solution of rhenium-platinum mixed metal clusters containing P-donor ligands... 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

Several mixed-metal clusters containing platinum and cobalt are known and some of them have been employed as methanol homologation catalysts.1 Among them, the title compound2 was first prepared unambiguously from the reaction of dichloro[l,2-ethanediylbis(diphenylphosphine)]platinum with sodium tetracarbonylcobaltate, Na[CO(CO)4]. The compound also may be prepared by the reaction of [l,2-ethanediylbis(diphenyl-phosphine)]bis(phenylethynyl)pIatinum with Co COJg.1... 

These heterogeneous catalysts consist of muitimetallic clusters, containing metals, such as platinum, iridium, or rhenium, supported on porous acidic oxide supports, such as alumina. The catalysts are said to be bifunctional because both the metal and the oxide play a part in the reactions. The metal is believed to carry out reversible dehydrogenation of paraffins to olefins, while the oxide is believed to carry out isomerization. 

An important consequence of the nonutilization of tangential orbitals is that platinum clusters often do not obey the normal electron counting rules and appear to be electron deficient (19,21,29,58,75,76). Electron counts are usually intermediate between those found in normal transition metal clusters (58-68) and those observed in gold clusters (58,78), but no satisfactory general electron counting theory has been developed for Pt-containing clusters. In small Pt clusters constructed from PtL2 units, theoretical studies have shown that the total electron count depends on the relative orientation of the... 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

Electron microscopy data on such catalysts show the average diameters of the metal clusters to be of the order of 10 A or lower. Clusters of this size necessarily consist almost exclusively of surface atoms. The stoichiometry of the strongly chemisorbed fraction thus appears to be close to one hydrogen atom per surface metal atom for platinum-iridium catalysts containing equal amounts of the two metals. If this stoichiometry were precisely correct, a value of H/M from Figure 4.20 would be a direct measure of the metal dispersion, that is, the ratio of surface atoms to total atoms in the metal clusters. 

Transition metals with odd atomic numbers cannot achieve the expected FAN in monomeric compounds. In cases where carbonyls of these elements are known, the compormds contain more than one metal atom and metal-metal bonds these effectively contribute one extra electron to each metal Mn2(CO)io, Co2(CO)s. Many polynuclear metal carbonyls are known. The largest metal cluster that has been characterized is a polynuclear platinum carbonyl anion,... 

Polynuclear anionic metal carbonyl compounds are usually prepared by reduction reactions of metal carbonyls M(CO) with such reducing agents as the alkali metals, NaBH4 in ethers, hydrocarbons, liquid ammonia, and similar solvents [see, for example, reactions (2.54), (2.55), (2.84), (2.89), (2.95>-(2.102), and (2.108)-(2.113)]. In alkali medium the metal carbonyls may be reduced by certain solvents (e.g., alcohols) or by the CO ligand itself, and in the presence of Lewis bases the carbonyls disproportionate to give anionic clusters. The mixed metal clusters containing platinum and rhodium are formed by reduction reactions of chloro complexes... 

Meanwhile very active and small crystallites have been obtained, for instance, by thermal decomposition and reduction with H2 of molecular clusters as [Pt3(CO)6]n n = 2, 3, 4, 5) [11] or (r -CsH5)2Ni2(CO)2 and (T7 4 5H5)3Ni3(CO)2 [12]. It appears that the size of metallic particles obtained by these procedures does not exceed the size of the original molecular metal cluster. However it has not been proved in the case of platinum that the metal particles contain the same number of metal atoms as the molecular clusters from which they are formed. In the case of nickel particles [12] spectroscopic and chemisorption evidence suggest that the supported nickel atoms retain the original geometrical disposition of the parent molecular cluster. Meanwhile a certain reversibility to reform a carbonyl cluster by reaction with CO has been proved. 

As befits current interest, the largest number of reviews centre on metal carbonyl cluster compounds. General topics covered included electron transfer reactions, ligand and cluster transformations, and the chemistry of metal clusters containing nitrosyl and nitrido ligands.More specific topics reviewed include sulphi do-osmium carbonyl cluster compounds,and homonuclear platinum clusters.The preparations of [Nb(C0)6] [M2(CO)io(m-H)]" (M = Cr or W), Mn2(C0)aX2 and Re C0)sX (X = Cl, Br or I) are described in Volume 23 of Inorganic Syntheses. ... 

---