The rational design of molecular systems that exhibit tunable optical and/or magnetic behavior as a function of external stimuli (temperature, electric or magnetic fields, light, pressure…), is the subject of intense world-wide research activity to conceive high performance molecule-based electronic devices. Promising applications are expected in numerous areas such as information processing, highdensity recording media, molecular switches, sensors and display devices.[1,2] Among known switchable molecular systems, tunable optical and magnetic properties have been reported in many cyanido-based materials like Prussian blue (PB) analogues.[1,2c] K. Hashimoto reported in 1996 the first example of Co/Fe PB analogue that shows photochemically controlled magnetic properties.[3] Following this seminal work, a large series of Co/Fe PBs was synthesized allowing a deep understanding of the electron-transfer properties.[4] These compounds contain {Fe(μ-CN)Co} motifs that exhibit a reversible metal-to-metal electron-transfer process which converts diamagnetic {FeII LS(μCN)CoIII LS} into paramagnetic {FeIII LS(μ-CN)CoII HS} pairs.[3,4] In the past 15 years, chemists have been exploring the synthesis of molecular magnetic and photoresponsive complexes through a rational choice of cyano-based building-blocks.[5] This approach has been extremely successful and various molecular architectures have been obtained with remarkable properties such as single-molecule magnet behavior,[6] spin crossover,[7] electron-transfer process[8] and photo-induced magnetism.[8b-f,9] Recently, our group reported tetranuclear cyanido-bridged {Fe2Co2} complexes that exhibit reversible thermally- and photo-induced intramolecular electron-transfer in the solid state.[8c] In order to probe if the bistability of {Fe2Co2} complexes can be transferred in a general manner to different solvents, we have oriented our effort towards more soluble compounds using functionalized bipyridine (bpyR ) ligands: {[(Tp*)Fe(CN)3]2[Co(bpyR )2]2}[X]2 (Tp* = tris(3,5-dimethyl)pyrazolyl borate, bpyR = 4,4’- R,R-2,2’-bipyridine with R = alkyl groups, X = a monoanion).[10] In this lecture, we will compare different {Fe2Co2} complexes that exhibits electron-transfer in solid state and in a broad range of solvents. This work demonstrates and generalizes the possibility to transfer the solid state properties of switchable {Fe2Co2} complexes to solutions, and thus opens up new possibilities to control molecular optical and magnetic behaviors by tuning the energy of the electron-transfer process through variations in solvent composition and polarity.