Pressure and temperature effects on metal-to-metal charge transfer in cyano-bridged CoIII–FeII complexes

Abstract

The effects of pressure and temperature on the energy (E_op) of the metal-to-metal charge transfer (MMCT, Fe^II→Co^III) transition of the cyano-bridged complexes trans-[L¹⁴CoNCFe(CN)₅]⁻ and cis-[L¹⁴CoNCFe(CN)₅]⁻ (where L¹⁴ = 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine) were examined. The changes in the redox potentials of the cobalt and iron metal centres with pressure and temperature were also examined and the results interpreted with Marcus–Hush theory. The observed redox reaction volumes can mainly be accounted for in terms of localised electrostriction effects. The shifts in E_op due to both pressure and temperature were found to be less than the shifts in the energy difference (ΔE°) between the Co^III–Fe^II and Co^II–Fe^III redox isomers. The pressure and temperature dependence of the reorganisational energy, as well as contributions arising from the different spin states of Co^II, are discussed in order to account for this trend. To study the effect of pressure on Co^III electronic absorption bands, a new cyano-bridged complex, trans-[L¹⁴CoNCCo(CN)₅], was prepared and characterised spectroscopically and structurally. X-Ray crystallography revealed this complex to be isostructural with trans-[L¹⁴CoNCFe(CN)₅]·5H₂O.