Charge-transfer interactions between bipyridinium ions and octacyano-molybdates and -tungstates

Abstract

Investigation of the interaction between the 4,4′-bipyridinium(2+) cation (4,4′-H₂bipy²⁺) and the octacyanometalates(IV)[M(CN)₈]^4–(Mo or W) led to the isolation of [4,4′-H₂bipy]₂[W(CN₈)]·4.5H₂O and its isomorphous molybdenum analogue. The spectroscopic properties of both and the X-ray structural characterization of the former [monoclinic, space group P2₁/c, a= 11.281(5), b= 9.198(5), c= 30.113(24)Å, β= 91.21(5)°, Z= 4] are described. The [W(CN)₈]^4– anion has an approximate square-antiprismatic D_4d configuration with average dimensions W–C 2.168, C–N 1.136 Å, W–C–N 177.0°. The structure consists of a three-dimensional hydrogen-bonded framework and cation–anion interaction occurs mainly through the water molecules. The intensely coloured solids (hydrated and anhydrous) and aqueous solutions are discussed in terms of the ion pairs which exhibit an outer-sphere electron transfer between adjacent redox sites. The correlation between the structures of the hydrated solids, the close contact ion pairs of the anhydrous salt and the solventseparated ion pairs (solution) are discussed.