A series of bimetallic, trigonal bipyramidal clusters of type {[Co(N-N)(2)](3)[Fe(CN)(6)](2)} are reported. The reaction of {Co(tmphen)(2)}(2+) with [Fe(CN)(6)](3-) in MeCN affords {[Co(tmphen)(2)](3)[Fe(CN)(6)](2)} (1). The cluster can exist in three different solid-state phases: a red crystalline phase, a blue solid phase obtained by exposure of the red crystals to moisture, and a red solid phase obtained by desolvation of the blue solid phase in vacuo. The properties of cluster 1 are extremely sensitive to both temperature and solvent content in each of these phases. Variable-temperature X-ray crystallography; Fe-57 Mossbauer, vibrational, and optical spectroscopies; and magnetochemicai studies were used to study the three phases of 1 and related compounds, Na{[Co(tmphen)(2)](3)[Fe(CN)(6)](2)}(CIO4)(2) (2), {[Co(bpy)(2)](3)[Fe(CN)(6)](2)}[Fe(CN)(6)](1/3) (3), and {[Ni(tmphen)(2)](3)[Fe(CN)(6)](2)} (4). The combined structural and spectroscopic investigation of 1-4 leads to the unambiguous conclusion that 1 can exist in different electronic isomeric forms, {(CO2COFe2II)-C-III-Fe-II} (1A), {Co-III- (Co2FeFeII)-Fe-II-Fe-III} (1B), and {(Co3Fe2III)-Fe-II} (1C), and that it can undergo a charge-transfer-induced spin transition (CTIST). This is the first time that such a phenomenon has been observed for a Co/Fe molecule.