Species platinum

The metal-metal interaction and conductivity increase with pressure using bulkier ammines increases the Pt-Pt distance. Although palladium-containing ions can be substituted for the platinum species, the optical properties and metal-metal interaction causing pronounced dichroism are... 

The products obtained from the Pt(PR3)2X2-CNCH3 reactions were dependent on the nature of the platinum species. Five-coordinate adducts, [Pt(PR3)2(CNCH3)2X]X, were isolated for the iodo and bromo complexes (R=Ph), although the latter was unstable and slowly lost isocyanide. The observation of five-coordination here is somewhat unusual, but since this report, it was also observed in a different situation (85), mentioned above. The more common observation was the isolation of four-coordinate species, implying the low stability of most five-coordinate complexes. Data on these reactions are summarized below [Eqs. (33, 34)]. 

In some cases, adsorbed foreign species may give rise to acceleration of a reaction. In the hterature, a few cases have been described where strong catalytic effects were observed on catalytically inactive electrodes when small amounts of platinum species that had by accident arrived in the electrolyte solution became adsorbed on their surface. 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

These observations were rationaUzed by invoking two parallel, [L]-dependent mechanisms for the catalysis, one via monomeric platinum species (Scheme 5-5)... 

Figure 4.11. Cesium ion initiated static SIMS spectra of platinum species on fresh auto-exhaust catalyst.

Homogeneous catalysts have been reported, which can oxidize methane to other functionalized products via C-H activation, involving an electrophilic substitution process. The conversion of methane into methyl bisulfate, using a platinum catalyst, in sulfuric acid, has been described. The researchers found that a bipyrimidine-based ligand could both stabilize and solubilize the cationic platinum species under the strong acidic conditions and TONs of >500 were observed (Equation (5)).13... 

The cluster, Ni4(CO)6[P(CH2CH2CN)3]4, has been obtained in low yield (ca. 5%) by the reaction of Ni(CO)4 with P(CH2CH2CN)3 in boiling methanol17, 183, and the structure of this compound has already been reported in Fig. 12. Whereas the nickel species, Ni(CO)2(PR3)2, are generally stable, the corresponding platinum species decompose spontaneously50 ... 

We went back and looked at that system, because we thought it had quite a bit of promise. It turns out that, with triphenyl phosphine, platinum(O) will attack the phosphorous phenyl bonds to produce red polymers which are phosphido-bridged platinum species. But we were more interested in the small alkyl phosphines, because we expect those to make the metal center even more reactive (being sterically unhindered and more basic). 

In view of excess phosphine being inhibitory in many catalytic reactions, it is surprising that the hydrophosphination reaction is not suppressed by the phosphines formed. In agreement with this, the product phosphine P(CH2CH2CN)3 is reluctant to form tetrakis(phosphine)platinum species, allowing the metal complex to be coordinatively unsaturated. Likewise, the... 

Contrary to previous reports suggesting colloidal metal as the active species in Pt-catalyzed hydrosilylations, the catalyst was found to be a monomeric platinum compound with silicon and carbon in the first coordination sphere.615 The platinum end product at excess olefin concentration contains only platinum-carbon bonds, whereas at high hydrosilane concentration, it is multinuclear and also contains platinum-silicon bonds. An explanation of the oxygen effect in hydrosilylation was also given to show that oxygen serves to disrupt multinuclear platinum species that are formed when poorly stabilizing olefins are employed. 

The relative amounts of all these Pt species vary as a function of the pH and the chloride concentration. Only platinum species with a coordinated water molecule are regarded to be reactive, because, in contrast to coordinated chloride or hydroxide, this ligand can be easily substituted by other donor molecules. Hydroxo species are formed as indicated in Eqs. (3)-(7) (34, 35), with [ds-Pt(NH3)2(OH)2] as the stable end product in basic solution (36). It should be noted, however, that this species very easily dimerizes and trimerizes at higher concentrations, producing ions such as [cis-Pt(NH3)2]2(jU.-OH)2 and [cis-Pt(NH3)2]3(ju,-OH)3, as has been proved with, e.g., 195Pt NMR spectroscopy (36a, b). Very recently, accurate pAa values have been presented for the (de)hy-dronation equilibria (36b) the pKa values have been added to Eqs. (5)—(7). Miller and House (36c) have accurately determined the kinetic parameters for the several hydrolysis reactions. They concluded that acid hydrolysis of cis-Pt in vivo is unlikely to proceed beyond [cis-Pt(NH3)2Cl(H20)]+. 

In blood plasma, the chloride ion concentration is sufficiently large (about 100 mAf) to prevent cis-Pt hydrolysis, and the neutral platinum species most likely crosses the cell membrane. Inside the cell the chloride ion concentration is much lower (about 4mM), which allows for hydrolysis (35, 37). Because water is a far better leaving group than chloride or hydroxide (38, 39), the aqua species are most likely the reactive form of cis-Pt in vivo. Thus hydrolysis is the rate-limiting step in the reaction of cis-Pt with biomolecules such as proteins, RNA, and DNA (40). 

Relevant to the results described in this chapter is the synthesis of the tungsten-platinum species 1 (5). In this reaction an ethylene molecule... 

The complete transfer of an alkylidyne group to the carbaborane cage was first observed with tungsten-platinum species. In acetone, the salts 2d... 

These form the largest single group of hetero-Pt clusters, with more than 40 crystal structures (Table V) reported. Their chemistry, particularly with regard to triosmium-platinum species, is the most developed of any hetero-Pt clusters. 

Schmidt et al. (139) postulated that in the presence of excess oxygen, platinum was transported as volatile oxides through the gas phase and boundary layer. This mechanism could not adequately explain the reconstruction observed far into the excess ammonia regime. It was suggested that under these and other conditions, other volatile platinum species formed. Moreover, these species might decompose by reaction in the boundary layer, leading eventually to the platinum replating itself. 

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