Electron spin resonance studies of the solution photochemistry of octacyanotungstate(V) ions at 77 and 293 K

Abstract

While the photolysis of aqueous solutions of [W^V(CN)₈]^3– in its charge-transfer-to-metal band proceeds largely (90%) through a photoredox process, ca. 10% proceeds through the transient [W^V(CN)₇]^2– which either becomes solvated rapidly to give [W^V(CN)₇(H₂O)]^2– or, in the presence of high concentrations of X^– ions (X = Cl, Br, N₃, or SCN) or O₂, reacts to give [W^V(CN)₇X]^3– or [W^V(CN)₇(O₂)]^2– respectively, characterised by their g-tensors. [W^V(CN)₇]^2– is detected directly only on photolysis at 77 K with g_⊥= 1.918, g_∥= 2.010, although it can be spin-trapped, while [W^V(CN)₇(H₂O)]^2– is detected after warming photolysed glasses to 293 K followed by recooling to 77 K (at 77 K g_⊥= 1.918, g_∥= 2.020, g_av.= 1.952; at 293 K g_iso.= 1.947). [W^V(CN)₇(H₂O)]^2– reacts with [W(CN)₈]^4– to give [(CN)₇W^V(CN)W^IV(CN)₇]^6– with g_iso.= 1.944, g_⊥= 1.883, and λ_max.= 1 350 nm, and with [Fe(CN)₆]^3– to give [(CN)₅Fe^II(CN)W^V(CN)₇]^6– with g_iso.= 1.9398. In confirmation, oxidation of [W^IV(CN)₇(H₂O)]^3– by Ce^IV or H₂O₂ yields initially [W^V(CN)₇(H₂O)]^2– which on standing is converted slowly to [(CN)₇W^V(CN)W^IV(CN)₇]^6–.