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(NH ₃ ) 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(NH ₃ ) ₄ ] ²⁺ ,[W(CN) ₈ ] ^3- } exhibits an outer-sphere metal-to-metal charge-transfer transition at 485 nm (ε = 49 ± 2 dm ³ mol ^-1 cm ^-1 ) with association constant K _A = 26 ± 3 dm ³ mol ^-1 at I = 0.45 mol dm ^-3 (KNO ₃ ) 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 KNO ₃ for the novel ion pair. The {[Pt(NH ₃ ) 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 α ² = 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(NH ₃ ) ₄ ] ₂ [W(CN) ₈ ][NO ₃ ]·2H ₂ O. The complex is paramagnetic (at 293 K g _iso = 1.97; at 77 K g _⊥ = 1.96, g _|| = 1.98).