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Reduction of silver amine complexes by carbon monoxide

University of British Columbia

The kinetics of the reduction of silver amine complexes in aqueous solution by carbon monoxide were investigated. For a number of amines including ethyl-, methyl-, diethyl- ethanol-, diethanolamine and some primary diamines, the rate law was found to be of the form: [formula omitted] (1) where L denotes the amine. These kinetics were interpreted in terms of the following mechanism. AgL₂⁺ + H₂O ⇌ L-Ag-OH + LH⁺ (Rapid) (2) L-Ag-OH + CO [symbol omitted] L-Ag-COOH (Rate-determining) (3) L-Ag-COOH + Ag(I) → Products (Rapid) (4) The rate constant of the rate-determining step (3) was found to be nearly independent of the nature of the amine molecule, L, coordinated to silver ion, using the basicity constants of the amines and dissociation constants of the corresponding silver amine complexes. The actual overall rate of the reaction varied with the nature of amine but this was attributable only to the different equilibrium concentrations of L-Ag-OH. The rate of this rate-determining bimolecular process was found to be surprisingly fast; k₂₅。= 5x10² mole ⁻¹. sec.⁻¹, ΔH*~ 9 Kcal. mole⁻¹ and ΔS*~ -15 e.u. The reduction of silver ion by CO in acidic or neutral media is known to be very slow and this can now be attributed to the base catalyzed nature of the reaction. Silver complexes of primary diamines (ethylenediamine, 1,3-diaminopropane, etc.) were reduced more slowly; this was attributed to the stabilization of mono-complexed silver (I) species by chelate formation. In the case of ammonia normal kinetics were observed at higher pH but at Lower pH the rate became second order in (Ag(I))and inversely second order in (NH₄⁺). This was attributed to competition between decomposition of the intermediate complex and its further reaction with another Ag(I) species to give metallic silver and carbon dioxide. Evidence for similar competition was found with two tertiary amines, i.e., triethylamine and triethanolamine.

Text

2011-12-02

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http://hdl.handle.net/2429/39470

Chemistry

University of British Columbia

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