Hydroxides platinum complexes

Diene platinum complexes such as (COD)PtCl2, (NBD)PtCl2 (NBD = norbor-nadiene) and (DPD)PtCl2 (DPD = dicyclopentadiene) easily react with nucleophilic reagents such as carboxylates, alkoxylates, amines and hydroxides to afford the addition products in high yields [8,114- 117]. For example, reactions are shown in eqs. (21.41)-<21.43). 

A dimethylplatinum(ii) complex with an SOs group-equipped dipyridylmethane as a bidentate chelate ligand undergoes protonation to afford the dimethyl(hydrido)platinum(iv) complex 965 accompanied by coordination of the sulfonate group (Scheme 119)/ Addition of OH regenerates the Pt(ii) complex. Reaction of the complex with O2 forms the dimethyl(hydroxide)platinum(iv) complex 966. 

Syntheses are generally carried out by dissolving both reactants in water at a pH of 4.6-6.S. Base, such as sodium hydroxide, is added if the system is too acidic. A 1.5-3-fold excess of the platimun reactant is usually employed. Because the platinum complex products are water soluble, the products were isolated using fractional precipitation or freeze drying. The excess uncomplexed polymer usually remains in solution if the reaction mixture is diluted with a water-soluble organic liquid such as ethanol or isopropanol until the polymer complex precipitates. 

Rate and equilibrium constant data, including substituent and isotope effects, for the reaction of [Pt(bpy)2]2+ with hydroxide, are all consistent with, and interpreted in terms of, reversible addition of the hydroxide to the coordinated 2,2 -bipyridyl (397). Equilibrium constants for addition of hydroxide to a series of platinum(II)-diimine cations [Pt(diimine)2]2+, the diimines being 2,2 -bipyridyl, 2,2 -bipyrazine, 3,3 -bipyridazine, and 2,2 -bipyrimidine, suggest that hydroxide adds at the 6 position of the coordinated ligand (398). Support for this covalent hydration mechanism for hydroxide attack at coordinated diimines comes from crystal structure determinations of binuclear mixed valence copper(I)/copper(II) complexes of 2-hydroxylated 1,10-phenanthroline and 2,2 -bipyridyl (399). 

Several methods are employed for the purification of the single platinum metal salts, for instance re-crystallization of Na2[Pt(Cl)6] with oxidative hydrolysis of the impurities of Pd, Rh, Ir, etc. which together with base-heavy metals are removed as insoluble hydroxides. Generally the metals are obtained, initially as sponge or powder, by conversion of their salts through thermal decomposition or by reduction of aqueous solutions of the ammonium-chloro complexes. Os which has been oxidized to the volatile 0s04 and Ru have to be reduced by H2. 

McHenry et al. (63) suggested that a special soluble platinum [which is a not fully identified complex containing Pt(IV) chloride and hydroxide complexed with alumina] may be active in aromatization. Its presence was denied in a reducing atmosphere (64). Electron spin resonance (ESR) and optical spectroscopy gave evidence, however, that a part of it still exists even after reduction (65, 66). 

Hydroxylamine plays the same part in the molecule as ammonia in ammino-platinum compounds, but the substances differ somewhat in chemical behaviour, for hydroxylamine is more readily eliminated than ammonia from the complex. Also, m-dihydroxylamino-dichloro-platinum is not obtained by the interaction of free hydroxylamine and potassium chloroplatinite, the method used for the preparation of cis-dichloro-diammino-platinum. Again, tetrammino-platinous hydroxide, [Pt(NH3)4](OH)2, is a very strong base and easily soluble in water, whilst tetrahydroxylamino-platinous hydroxide, [Pt(NH2OH)4](OH)2, is almost insoluble in water and a comparatively weak base.2 For this reason Werner 3 suggested a different formula for the two substances, and indicated that possibly in the tetrahydroxylamino-com-pound the co-ordination number of the metal is six and not four, as in the tetrammino-compounds thus ... 

Hydroxy-bridged complexes [Pt2(,u-OH)2(PEt3)4]2+ can also be prepared. The structure consists of two square planar platinum(II) centers bridged by hydroxide ligands with an angle of 36.4° between the mean plane normals.1569 A useful method to prepare these complexes involves the use of phase-transfer catalysis with crown ethers to facilitate the reaction of KOH with platinum(II) chloro complexes.1570... 

The base hydrolyses of PtCl(NH3) + and c/s-PtCl2(NH3)4+ are possibly examples of nucleophilic substitution reactions 204 these remain the only cases which have not been proven otherwise. These reactions follow a rate law which is first order in the platinum(IV) complex, and a hydroxide ion dependence which is intermediate between zero and first order. The reaction is not catalyzed by Pt(NH3)2+. 

The next major theory of metal ammines was proposed by Carl Ernst Claus (1796—1864). In 1854, Claus rejected the ammonium theory and suggested a return to Berzelius view of complexes as conjugated compounds. He compared the platinum ammines not with ammonium salts nor with ammonium hydroxide but with metal oxides. He designated the coordinated ammonia molecule as passive, in contrast to the active, alkaline state in the ammonium salts, where it can easily be detected and replaced by other bases . 

---