Lithium pentacyanocobaltate coordination polymers with the formulas Li-3[Co(CN)(5)]. 4DMF (1, DMF = N,N-dimethylformamide) and Li-3[Co(CN)(5)]. 2DMF (2) have been synthesized and structurally characterized. Both compounds react reversibly with O-2, forming 1:1 superoxo complexes. Compound 2 reacts more rapidly and has the highest reported capacity for a solid O-2 carrier (55 std cm(3)/g). In contrast to previously reported dioxygen complexes of cyanocobaltates, the novel ability of 1 and 2 to react reversibly with O-2 is made possible by the interaction of Li cations with the nucleophilic N atoms of the cyanide ligands. This interaction effectively reduces the basicity of the cyanide ligands, decreasing the net electron density on Co(2+) and hence weakening the charge-transfer interaction with O-2. The structure of 1 was determined from single-crystal X-ray diffraction measurements. It crystallizes in the triclinic space group P (1) over bar (no. 2) with a = 11.568(7) Angstrom, b = 14.244(3) Angstrom, c = 8.118(3) Angstrom, alpha 97.44(2)degrees, beta = 103.00(4)degrees, gamma = 94.64(3)degrees, and Z = 2. Monocrystals of 2 suitable for X-ray structural analysis could not be grown, but the structure was solved ab initio from synchrotron powder diffraction data by using a simulated-annealing technique. The monoclinic cell. of 2 in space group P2(1)/n (no. 14) has cell parameters a 15.4455(8) Angstrom,b = 8.4216(4) Angstrom, c = 14.1505(8) Angstrom, beta = 97.780(3)degrees, and Z = 4.