The self-assembly of [Co(MeTPyA)(CH3COO)]PF6 (1) and [Fe(bbp)(CN)3]2– affords a cyanido-bridged square-shaped {Fe2Co2} tetranuclear complex, [{Co(MeTPyA)(μ2-NC)2Fe(bbp)(CN)}2]·3H2O (2; MeTPyA = tris((3,5-dimethylpyrazol-1-yl)methyl)amine and H2bbp = bis(2-benzimidazolyl)pyridine). The possibility of inducing an intramolecular electron transfer coupled spin transition in 2 by employing protonation as an external stimulant is explored. UV–visible spectrophotometric measurements, electrochemical and 1H NMR studies establish that a reversible intramolecular electron transfer coupled spin transition can be triggered in 2 upon addition of either acid or base.