A cyanide-bridged tetranuclear complex, [Co2Fe2(CM6-(tP*)(2)(dmbPY)(4)](PF6)(2)center dot 4CH(3)CN (1 center dot 4CH(3)CN) (tp*: hydrotris-(3,5-dimethylpyrazol-1-yl)borate and dmbpy: 5,5'-dimethyl-2,2'-bipyridine), was synthesized and its magnetic properties were investigated. Magnetic susceptibility measurements revealed that 1 exhibited an abrupt one-step thermal phase transition based on electron-transfer-coupled spin transition (ETCST) and light-induced ETCST at low temperature.