New binary ligands consisting of 2,2':6',2 ''-terpyridine and 2,2'-bipyridine were synthesized through the linkage of various lengths of the polymethylene chain (1a-1d; n = 3-6, where n is the number of CH2 units). Their Co(II) complexes, 2a-2d (Cl complexes) and 2a'-2d' (H2O complexes), were synthesized. They exclusively formed mononuclear complexes, which could be oxidized to Co(III) complexes in aqueous solutions after prolonged exposure to the air. The single crystals of 2b and Co(III) complex 2c '' were obtained, and their crystal structures were determined. Cyclic voltammograms of the Co(II) complexes showed the redox processes of the terpyridine, Co(II)/Co(I), and Co(III)/Co(II). The redox waves of Co(II)/Co(I) in 2a and 2a' were slightly different from those of other complexes because of the short polymethylene chain.