Photoexcitation of potassium octacyanotungstate(IV) and 2,2′-bipyridyl: kinetics and mechanism

Abstract

The reaction kinetics of photochemically generated [W(CN)7OH]4− in basic medium and [W(CN)6(CNH)-(H2O)]2− in acidic medium with 2,2′-bipyridyl have been studied in buffer solutions of pH 4.2–10.6, ionic strength 8.0×10−2 M at 20°C. The quantum yield for formation of the photoproduct was found to depend upon the pH, ligand and [W(CN)8]4− concentrations. The pseudo-first order rate constant and quantum yield in acidic medium are higher than in basic medium, due to weakening of the W−CN σ bond when the nitrogen of the cyano group in the complex is protonated. The mechanisms associated with the photosubstitution are different in the two media. The ionic strength does not affect the reaction rate, implying that the mechanism is not dissociative.