The flash photolysis of di-μ-oxo-bis(oxo-molybdate(V)) acid aqueous solutions in the presence of excess thiocyanate

The flash photolysis of the mono and di-thiocyanate complexes of μ-oxo bis(oxo-molybdenum(V)) (Mo₂O₄(NCS)_x(H₂O)_6?x^(2?x)+ where x = 1, 2) has been studied in the system composed of perchloric acid solutions of Mo₂O₄(H₂O)₆²⁺ in the presence of excess thiocyanate. Irradiation with light in the wavelength range 250–340 nm induced the formation of three reaction intermediates identified as μ-oxo-bis (oxo-molybdenum(V)) species: Mo₂O₃(H₂O)₈⁴⁺ (A), Mo₂O₃(NCS) (H₂O)₇³⁺ (B), and Mo₂O₃(NCS)₂(H₂O)₆²⁺ (C). A rapid equilibria between the transient species is established before they decay following a first order kinetics. Intermediate A is the only species effective in promoting the observed decay leading to the end products Mo(VI) and H₂. The relative absorption spectrum of species A, B, and C are reported, as well as the measured stability constants for the formation of B and C in the temperature range 20–45°C. An overall mechanism is proposed.