The valence states of the nucleogenic Fe-57 arising from the nuclear disintegration of radioactive Co-57 by electron capture decay, Co-57(EC)Fe-57, have been studied by Mossbauer emission spectroscopy (MES) in the Co-57-labeled systems: [Co-57/Co(terpy)(2)]Cl-2. 5H(2)O (1), [Co-57/Co(terpy)(2)](ClO4)(2).1/2 H2O (2), and [Co-57/Mn(terpy)(2)](ClO4)(2). 1/2 H2O (3) (terpy = 2,2':6',2 " -terpyridine). The compounds 1, 2, and 3 were labeled with ca. 1 mCi of Co-57 and were used as the Mossbauer sources at variable temperatures between 300 K and ca. 3 K. [Fe(terpy)(2)]X-2 is a diamagnetic low-spin (LS) complex, independent of the nature of the anion X, while [Co(terpy)(2)]X-2 complexes show gradual spin transition as the temperature is varied. The Co(II) ion in 1 "feels" a somewhat stronger ligand field than that in 2; as a result, 83% of 1 stays in the LS state at 321 K, while in 2 the high-spin (HS) state dominates at 320 K and converts gradually to the LS state with a transition temperature of T-1/2 approximate to 180 K. Variable-temperature Mossbauer emission spectra for 1, 2, and 3 showed only LS-Fe-57(II) species at 295 K. On lowering the temperature, metastable HS Fe(II) species generated by the Co-57(EC)Fe-57 process start to grow at ca. 100 K in 1, at ca. 200 K in 2, and at ca. 250 K in 3, reaching maximum values of 0.3 at 20 K in 1, 0.8 at 50 K in 2, and 0.86 at 100 K in 3, respectively. The lifetime of the metastable HS states correlates with the local ligand field strength, and this is in line with the "inverse energy gap law" already successfully applied in LIESST relaxation studies.