PHOTOEXCITATION OF POTASSIUM OCTACYANOMOLYBDATE(IV) WITH 1,10-PHENANTHROLINE AND 2,2'-BIPYRIDYL - KINETICS AND MECHANISM OF THE REACTION IN ACIDIC MEDIUM

The kinetics of thermal reactions of photochemically generated [Mo(CN)6(CNH)(H2O)]2-with 1,10-phenanthroline and 2,2'-bipyridyl in acidic medium were studied in buffer solutions of pH 4.2-7.0 and ionic strength 8.0 x 10(-2) M at 25-degrees-C. The quantum yield of formation of the photoproduct was calculated and was found to depend on the pH, ligand and [Mo(CN)8]4- concentration. A reaction scheme for the photochemical and thermal reactions of octacyanomolybdate(IV) ion with 1,10-phenanthroline and 2,2'-bipyridyl is proposed. The rate of the reaction and the quantum yield increase with an increase in hydrogen ion and ligand concentration and decrease with an increase in [Mo(CN)8]4- concentration. In acidic medium the excited species, [Mo(CN)8]4-*, protonated at the cyano group which causes the electron density to shift from the Mo-CN bond axis towards the cyano group. On protonation the pi and sigma bonds of Mo-CN behave oppositely, i.e. the former becomes stronger and the latter becomes weaker leading to elongation and consequent weakening of the Mo-CN bond. Therefore the rate of reaction and the quantum yield increase at lower pH.