σ-Bonded organotransition-metal ions. Part XVI. Oxidation, complex formation, and electrophilic substitution in the reactions of tricarbonyl(η-cyclopentadienyl)(pyridiniomethyl)-molybdenum and -tungsten with thallium(III) and mercury(II) species in aqueous solution

Abstract

The reaction of tricarbonyl(η-cyclopentadienyl)(pyridiniomethyl)-molybdenum and -tungsten with thallium(III) perchlorate in aqueous acidic solution involves a rapid (probably two-electron) oxidation followed by a series of reactions leading to the corresponding pyridinioacetic acid as the main organic product. The marked increase in the rate of oxidation as the acid concentration is decreased is ascribed to the reactivity order [Tl(OH)₂]⁺[Tl(OH)]²⁺ Tl³⁺. These reactions are compared with the corresponding oxidations induced by the hexachloroiridate(IV) ion. The reaction of dilute solutions of the same substrates with mercury(II) perchlorate in aqueous acidic solution involves initial rapid but reversible co-ordination of the substrate to the mercury(II) species. In the case of the molybdenum substrates, the final organometallic product is the corresponding pyridiniomethylmercury(II) ion. The rates of reaction of these substrates are indicative of a normal bimolecular electrophilic-substitution reaction between the mercury(II) species and the uncomplexed substrate. The influence of chloride ion on the above reactions is briefly explored and it is noted that a different course is taken in the reactions with the mercury(II) species when higher concentrations of reagents are employed.