The design of molecule-based systems displaying tunable optical and/or magnetic properties under external stimuli received a great deal of attention in the past few years. This interest is driven by the potential applications in high-performance molecule-based electronics. As an example, 3D Fe/Co Prussian blue compounds exhibit a concomitant change in magnetic and optical properties due to a temperature- or light-induced metal-to-metal electron transfer (ET). The foregoing remarkable properties in Prussian blues prompted us to design soluble molecular fragments of these coordination networks through a rational building-block approach in order to mimic their properties on a single molecule.[1-4] With a judicious choice of the ligands for metal ion precursors, we prepared a octanuclear,[1] tetranuclear[2] and recently dinuclear[3] cyanido-bridged Fe/Co complexes. In the solid state, while an intramolecular ET is observed for the [Co4Fe4] and [Co2Fe2] complexes,[1,2] the Co ion of our first dinuclear complex exhibits a spin crossover (SCO) involving a [FeIIILS-CN-CoIILS] ground state and a thermally populated [FeIIILS-CN-CoIIHS] state.[3] To our knowledge, this compound is the only example of a heterobimetallic complex exhibiting a CoII SCO. Remarkably, our studies of these [ConFen] complexes in solution reveal important optical and magnetic changes induced by an intramolecular metal-to-metal ET triggered and modulated by a controlled protonation of the complex, by the solvent nature or by temperature. Therefore, these molecules act as different molecular switches depending on their physical state and external stimuli.[3] These results motivated us to design new dinuclear [FeCo] complexes exhibiting both thermally and light induced electron transfer in solid state. Learning from these previous systems, new dinuclear complexes[4] have been designed by a rational building-block approach. Combined structural, spectroscopic, magnetic and photomagnetic studies reveal that a metal-to-metal electron transfer that can be triggered by light, temperature and lattice contents is observed for the first time in solid state for a dinuclear cyanido-bridged Fe/Co complex (See figure).[4]figure