New donor–acceptor system based on [Pt(NH₃)₄]²⁺ and [W(CN)₈]^3- ions

Abstract

A new donor–acceptor system based on [Pt(NH₃)₄]²⁺ and [W(CN)₈]^3- has been characterized in aqueous solution and the solid state by spectroscopic and electrochemical techniques. In aqueous solution the 1∶1 ion pair {[Pt(NH₃)₄]²⁺,[W(CN)₈ ]^3-} exhibits an outer-sphere metal-to-metal charge-transfer transition at 485 nm (ε = 49 ± 2 dm³ mol^-1 cm^-1) with association constant K_A = 26 ± 3 dm³ mol^-1 at I = 0.45 mol dm^-3 (KNO₃) and 298 K. Electrochemical measurements revealed a [W(CN)₈]^3-/4- reversible couple at +0.27 V (vs. saturated calomel electrode) in 0.1 mol dm^-3 KNO₃ for the novel ion pair. The {[Pt(NH₃)₄]²⁺,[W(CN)₈ ]^3-} system is paramagnetic in aqueous solution (g_iso = 1.96 at 293 K). The results are interpreted using Hush theory (coupling parameters α² = 1.69 × 10 ^-4 and H_if = 268 cm^-1, reorganization energy for the optical electron-transfer process χ = 164.6 kJ mol^-1, thermal activation barrier E_th = 92.4 kJ mol^-1 and electron-transfer rate constant k_et = 3.3 × 10 ^-4 s^-1) and compared with the corresponding parameters for other [M(CN)_n]^3-/4- couples. The bimetallic Pt–W system has been isolated in the solid state as [Pt(NH₃)₄]₂[W(CN)₈][NO ₃]·2H₂O. The complex is paramagnetic (at 293 K g_iso = 1.97; at 77 K g_⊥ = 1.96, g_|| = 1.98).

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