A [Co(N)(2)(O)(2)(S)(2)]-type complex containing two coordinated thioether groups, trans(O)-[Co(edtda)(en)](+) (edtda = ethylenedithiodiacetate; en = ethylenediamine), has been prepared which was spontaneously resolved. The photodecarboxylation reaction of the trans(a)-[Co(edtda)(en)](+) provides trans(C,O)-[Co(CH2S(CH2)(2)SCH2COO)(en)](+), which has a -SCoCH2 three-membered ring. The crystal structures, UV/vis absorption, H-1 NMR spectra, and chiral inversion reaction of these complexes are discussed in connection with their stereochemistry.