The synthesis and characterization of cobalt(II) polypyridine complexes of general formula [Co(terpy)(B)X]X is described, where terpy = 2,2’:6’,2"-terpyridine, B = 2,2’-bipyridine (bpy), 1,10-phenanthroline, or various ring-substituted phenanthrolines, and X- is a weakly binding ligand or counteranion. The complexes react reversibly with dioxygen in polar nonaqueous solvents forming 2:1 CO to O2 adducts, which exist as mu-peroxo dimers. The structure of [[Co(terpy)(bpy)]2(mu-O2)](PF6)4.2CH3COCH3 (2a) was determined by single-crystal X-ray crystallography : crystal space group P1BAR with a = 10.953(2) angstrom, b = 12.509(3) angstrom, c = 14.426(3) angstrom, alpha = 105.15(2)degrees, beta = 103.41(2)degrees, and gamma = 110.39(2)degrees. In the cation of 2a the O-O distance for the bridging peroxo ligand (1.419(7) angstrom) is at the short end of the usual range (1.41-1.49 angstrom). Formation constants for the Co(II) polypyridine complexes’ O2 adducts were determined electrochemically and were found to be dependent on the donor properties of the ligand B, the anion X, and on the solvent. In nitrobenzene solution, these dioxygenation constants are significantly reduced by the addition of X = triflate and X = 4-chloro-3-nitrobenzenesulfonate ligands which compete for the O2-coordination site. The presence of a coordinated triflate in [Co(terpy)(3,4,7,8-tetramethyl-1,10-phenanthroline)CF3SO3]CF3SO3 (7) was demonstrated by single-crystal X-ray crystallography : for C33H27CoF6N5O6S2 (7) space group P2(1)/c, with a = 8.827(2) angstrom, b = 20.815(5) angstrom, c = 19.121(6) angstrom, beta = 98.80(2)degrees, and Z = 4. Intensity data were taken on a Nicolet R3m diffractometer in the range of 4-degrees less-than-or-equal-to 2theta less-than-or-equal-to 50-degrees. A total of 6525 (6135 independent) reflections were collected; of these 3769 had F(o) > 5sigma(F(o)). The structure was solved by direct methods and refined to R(F) = 5.13% and R(w)(F) = 5.40%. Hydrogen atom contributions with D(C-H) = 0.96 angstrom were included. Further details are given in Tables 3 and 4 and in the supplementary material. The cation (Figure 2) consists of a Co(II) ion coordinated by molecules of tetramethyl-1,10-phenanthroline (bidentate) and terpyridine (tridentate) and an O-bound triflate ion for which D(Co-O) = 2.165(3) angstrom. The various synthesized Co(II) polypyridine complexes are relatively long-lived reversible dioxygen carriers. Solutions of [[Co(terpy)(4,7-diphenyl-1,10-phenanthroline)]2-(mu-O2)]triflate)4 (5) (0.1 M) under O2 were found to retain 60% of their O2 activity after 60 days at 25-degrees-C (a projected half-life of 8 months). A method was found for partially regenerating the spent dioxygen carriers. Heating the spent oxygen carrier solutions under N2 to 150-degrees-C followed by cooling in air resulted in their partial regeneration to O2-active species.