Platinum complexes thiourea reaction

Two examples of aquation/anation studies of chloro-platinum(II) complexes of possible medical relevance appeared in subsection 1 above 202,207). Aquation of cisplatin is slower in the presence of DNA but not in the presence of phosphate 220). DNA also inhibits substitution in [Pt(terpy)(py)]2+ and related complexes. For reaction of these charged complexes with iodide ion inhibition is attributable to electrostatic interactions - the complex is concentrated on the double helix and thus separated from the iodide, which distances itself from the helix. Intercalation of these complexes within the helix also serves to make nucleophilic approach by neutral reagents such as thiourea more difficult 221). 

One or two minor points may be added to Table 8 above the adducts of aldehydes and ketones to palladium and platinum complexes react with SO2, CO2 and NO2 with displacement of the organic moiety to give the products expected for addition of these molecules to a dioxygen complex diphenylacetylene displaces both the carbonyl and the dloxygen ligand Adducts of CS2 and thiourea have also been reported The products obtained with NO2 depend on the reaction conditions, especially the solvent reaction of NO2 with (r-BuNC)2M02 gives a trans dinitrate for M = Ni and a cis dinitrate for M = Pd ). 

A useful method to probe whether the reaction mechanism involves an associative or dissociative pathway is to measure AV (the volume of activation) for the reaction. High pressure kinetics in methanol give AV 1 —12 cm3 mol-1 for an associative first step, and +7.7 cm3 mol"1 for the isomerization reaction. It is proposed that the faster reaction is a solvolytic replacement of Cl" followed by a dissociative isomerization step with [PtR(MeOH)(PEt3)2]+ (R = alkyl, aryl equation 210).580 Since isomerization and substitution reactions are mechanistically intertwined, it is useful to note here that for the rates of substitution of both cis- and frara,-PtBr(2,4,6-Me3C6H2)(PEt3)2 by I" and thiourea, the volumes of activation are negative, in support of associative processes.581 Further support for associative solvation as the first step in the isomerization of aryl platinum(II) complexes has been presented,582 and the arguments in favor summarized.583... 

Entering Groups. Rates of reaction of [PtCl4], [PtCl3(OH2)], and [PtCl2(OH2)2] with ethene are all rather similar. Rates and activation parameters for the reaction of a variety of uncharged platinum(ii) complexes with cyanide have been reported. The complexes in question are [PtCl(N02)(NH3)2], [Pt(N02)2(NH3>2], [Pt(CN)a-(en)], and rra s -[Pt(CN)(N02)(NH3)2] cyanide substitution follows the usual rate law [equation (1) above]. The variation of the second-order term with solvent composition for the reaction of [PtCl2(bipy)] with thiourea in aqueous dioxan and in aqueous THE has been discussed in terms of solvent effects on the initial and transition states (see Chapter 5 of this Part). ... 

The increase in electrophilicity of coordinated ligands commonly leads to more favorable hydrolysis reactions. There are many reports on hydrolysis reactions of coordinated carboxamides, halogenated alkylamines, Schiff bases, thioamides, nitriles, etc. For example, thiourea and thioacetamide complexes of platinum metals decompose on heating in basic solution with formation of the corresponding metal sulfides, Eq. 1.26 and 1.27 ... 

Thiourea acts as a mild reductant (E° = 0.427 V) for transition metal complexes and is oxidized to formamidine disulfide. Thiourea is an excellent ligand for many metal ions, especially for those of the platinum metals.88 Therefore, in most of its redox reactions, thiourea acts simultaneously as a reductant and as a ligand. For example, OSO4 reacts with thiourea in sulfuric acid to form [Osfthiole] and formamidine disulfide.89 Addition of [NH4][Tc04] to a solution of thiourea in a mixture of ethanol and hydrochloric acid results in the formation of the homoleptic thiourea complex of technetium(in), [Tc(thio)6] ". An X-ray structure study of the isolated product, [Tc(thio)6]Cl3-4H20, has shown that the six thiourea ligands are S-bonded to the metal. 

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