Spectroscopic data, platinum

A number of comparative studies have been made of organogold compounds and related complexes of neighboring metals, notably platinum and mercury as well as the other group IB elements. These comparisons have included such aspects of their chemistry as thermal stability and reactivity, bonding modes, and spectroscopic data. 

Preparing trans-hydrido-alkyls or -aryls of platinum(II) does not require prior isolation of a hydroxo complex. The main routes we have used are shown in Equations 9-11, and selected spectroscopic data, including some from the literature, are in Table IV. 

All the compounds are crystalline solids, and with the exception of RuH(OCOCH3)(CO)[P(C6H5)3]2, can be manipulated in air briefly without decomposition. The platinum complex, ris[Pt(OCOCH3 )2 P(C 6H 5 )3 2] is soluble in chloroform, dichloromethane, and methanol but practically insoluble in benzene and acetone. The other complexes are soluble in chloroform and dichloromethane, moderately soluble in benzene and acetone, and almost insoluble in light alcohols. Analytical and spectroscopic data are given in the following table. Infrared data refer to mulls in Nujol XH nmr data were obtained at 90 MHz using solutions in chloroform-d and are referenced to TMS. 

The partial confusion arising after Dewar s and Chatt s reviews were published, was resolved after Chatt and Duncanson reported in 1953 in the Journal of the Chemical Society the results of infrared spectroscopic studies on a range of olefin platinum(II) complexes [38]. In this highly cited paper they proposed, with particular reference to Dewar s model, that in the olefin platinum(II) complexes the cr-type bond would be formed by overlap of the filled re-orbital of the olefin with a vacant 5d6s6p2 hybrid orbital of the platinum atom, and the re-type bond by overlap of a filled 5d6p hybrid orbital of the metal with the empty antibonding re-orbital of the olefin (Fig. 7.8). In addition, Chatt and Duncanson illustrated how the model could be used to interpret not only the physical properties of the olefin platinum compounds, such as the spectroscopic data and dipole moments, but also their reactivity and their greater stability compared to the olefin silver salts. 

First in-situ infrared investigations of phosphoric acid adsorption on platinum and gold were performed in HCIO4 as base electrolyte [138]. More recently, spectroscopic data in alkaline solutions were reported [37, 158]. However, not enough attention was paid in these studies to the problem of acid-base equilibrium displacements in solution and to the overlapping of solution and surface features which make the interpretation of spectra very difficult. Results on the adsorption of phosphate species on polycrystalline platinum at pH 2.8 (79% H2PO4 and 21 % H3PO4) are shown in Fig. 58a [146]. 

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

Oxidation with lead dioxide in 5% phosphoric acid yielded acetaldehyde and formaldehyde. Catalytic reduction in the presence of platinum resulted in the absorption of 1.5 mole equivalents of hydrogen and the isolation, by its steam volatility, of nearly 0.6 mole of a-methylbutyric acid. The carbon skeleton of sarracinic acid was thus determined, and the accompanying hydrogenolysis (some nonvolatile acid was also obtained) established that the double bond and hydroxyl constituted an allylic alcohol moiety. Since further data (spectroscopic data would be especially valuable) were not available, Danilova and Kuzovkov (127) were limited to the conclusion that sarracinic acid could be represented by one of three possible structures (CLXIa-c) ... 

Several techniques have been reported to yield particle size and distribution profiles, but their use has not been generalized yet, either because the technique is not straightforward or limited to specific cases or because the method used to process the spectroscopic data is too involved. This can be exemplified by the work of Dalla Betta and Boudart (188a), who determined by infrared (IR) the particle size of platinum encaged in CaY zeolite. The size was deduced from the fraction of OH groups exchanged with deuterium, and it was assumed that the isotopic exchange at low temperature is rapid only in the immediate vicinity of the particle. This... 

The selectivity for VA may be explained, in part, by the relative energies of the orbitals on the olefins and platinum. Meester et al. have reported orbital energies for platinum, VA and methyl acrylate in trans-PtClo(olefin)(pyridine) conplexes based on spectroscopic data (2i). Their results indicate a closer match for methyl acrylate in the energies of the unfilled olefin and platinum orbitals, but a closer match for VA in the energies of the filled orbitals. Apparently, the o-bond interaction predominates in determining the relative reaction rates (and product stabilities) for these olefin substitutions. The fact that methylmethacrylate does not react at all is probably due to steric hindrance of the additional methyl group on the double bond. A similar argument can... 

To summarize these results, it becomes now clear that EMIR Spectroscopy is particularly well suited to follow the fate of the different small adsorbed organic residues, resulting from the chemisorption of small organic molecules, such as CH3OH. The nature and the quantity of adsorbed species depend strongly on the structure of the catalytic surface, on the concentration of methanol in solution, on the adsorption time, on the applied electrode potential,... All these various experimental conditions lead to a great variety of adsorbed species, and control their surface distribution. According to these spectroscopic data, the reaction oxidation mechanisms of methanol adsorption and oxidation at platinum electrodes... 

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