Photoexcitation of octacyano complexes of molybdenum(IV) and tungsten(IV) with ethylene diamine. Kinetics and reaction mechanism

Abstract

The kinetics of thermal reactions of photochemically generated aquaheptacyano MoIV and WIV complexes and their protonated and deprotonated forms have been studied spectrophotometrically in buffer solutions at pH 5.0–10.0, and ionic strength, 8×10−2 M at 25°C. A reaction scheme for the photochemical and thermal reactions of the MoIV and WIV octacyano complexes with ethylene diamine is proposed. Rate constants and quantum yields for these systems are maximal at pH 8.0. At pH>8.0, the reverse reaction, generating octacyano complexes from heptacyano species, is faster; at low pH the ligand is protonated and is less reactive. Quantum yields are higher for Mo than for W owing to the shorter life time of excited state species. This is because physical deactivation is expected to be more rapid in the heavier element due to enhanced spin-orbit coupling. Furthermore Mo-induced splitting is larger in [W(CN)8]4− as compared to [Mo(CN)8]4− which results in greater bond strength for tungsten.