Platinum aqua-ions

Syntheses of palladium and platinum aqua ions [42] include... 

Aqua ions are known but not very stable. Substitution of Pt in aqueous solution is sometimes zero-order in the added ligand, L, or can have both L-dependent and L-independent contributions to the rate, probably because intermediate formation of an unstable aqua complex is the rate-determining step for the L-independent pathway. A large number of O-donors, particularly anionic ones, give stable complexes, for example, carbonate, acetate, oxalate, acetylacetonate, and alkoxide. Tetrameric platinum(II) acetate is formed by formic acid reduction of Pt solutions in acetic acid. It does not appear to be a very useful synthetic precursor for Pt chemistry. The acetylacetonate [Pt(acac)2] is monomeric and square planar. 

As may be seen from the potential-pH diagram " (Fig. 6.3) platinum is immune from attack at almost all pH levels. Only in very concentrated acid solutions at high redox potentials (i.e. under oxidising conditions) is there a zone of corrosion. This accounts for the solubility of platinum in aqua regia. Platinum is also prone to complex-ion formation, and this can lead... 

Bond and coworkers [521] have described Au(III) electroreduction and Au(0) oxidation stripping processes in dilute aqua regia utilizing platinum, rhodium, iridium, gold, and glassy carbon electrodes. Sorption of tetrachloroaureate ions on carbon paste electrode modified with montmorillonite has been performed as a preconcentration step in the determination of gold in pharmaceutical preparation [522]. 

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). 

Ferrocene is only one of a large number of compounds of transition metals with the cyclopentadienyl anion. Other metals that form sandwich-type structures similar to ferrocene include nickel, titanium, cobalt, ruthenium, zirconium, and osmium. The stability of metallocenes varies greatly with the metal and its oxidation state ferrocene, ruthenocene, and osmocene are particularly stable because in each the metal achieves the electronic configuration of an inert gas. Almost the ultimate in resistance to oxidative attack is reached in (C5H5)2Co , cobalticinium ion, which can be recovered from boiling aqua regia (a mixture of concentrated nitric and hydrochloric acids named for its ability to dissolve platinum and gold). In cobalticinium ion, the metal has the 18 outer-shell electrons characteristic of krypton. 

It is a noble metal, not attacked by dilute or concentrated acids, except aqua regia, which dissolves platinum when hexachloroplatinate(IV) ions are formed 3Pt + 4HN03 + 18HC1 - 3[PtCl6]2 +4NO +6H+ +8H20 Molten alkalis and alkali peroxides attack platinum, therefore these should never be melted in platinum crucibles. In its compounds, platinum can be mono-, di-, tri-, tetra- and hexavalent, tetravelent platinum being the most important in analytical practice. 

Platinum is not attacked by any of the mineral acids, but does dissolve in aqua regia, an HCl HNO3 mixture. The nitric acid provides the oxidizing power and the HCl provides the chloro ligands that stabilize Pt in solution. The metal is also attacked by fused alkalis, by fused sodium peroxide and, at red heat, by chlorine and fluorine, especially in the presence of NaCl or other halide ion sources. 

The solution of platinum in aqua regia likewise results in the stable hexachloroplatinate ion, PtCl,. . ... 

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