The factors controlling the reversible dioxygen binding in lacunar cobalt(II) cyclidene complexes (Figure 1) have been examined. Extensive structural variations reveal that the dioxygen affinity can be controlled by both steric and electronic means, The dioxygen affinity decreases monotonically with the length of a polymethylene bridging group (R1, Figure 1) from octamethylene to tetramethylene; no binding occurs with the still shorter trimethylene bridge. From the analysis of a large array of experimental data, the effects of the R2 and R3 substituents on dioxygen affinity are found to be mainly electronic; for example, electron-withdrawing groups at the R2 and R3 positions decrease the affinity. The various substituent effects are cumulative, but they am not additive. The X-ray crystal structure of the dioxygen adduct [Co(MeMeC6[16]cyclidene)(1-MeIm)(02)](PF6)2 provides significant insight into the structural relationships. The space group is Pnma with a = 23.226(2) angstrom, b = 18.981(2) angstrom, and c = 10. 124(1) angstrom. The Co-O-O bond angle is 121(1)-degrees, and the 0-O distance is 1.32(2) angstrom. The conformation of the ligand in this six-coordinate cobalt complex is different from that of the rive-coordinate complex, as expected on the basis of structures in which isothiocyanate occupies the cavity. The structural influence of the coordination of a small molecule within the cyclidene cavity is further explored by examination of the crystal structure of the cobalt(III) complex [Co(MeMeC8[16]cyclidene)(SCN)2](PF6). The space group is P1BAR with a = 12.020(3) angstrom, b = 12.379(3) angstrom, c = 14.159(4) angstrom, alpha = 111.73(2)-degrees, beta = 99.49(2)-degrees, and gamma = 94.57-degrees. Again, a predicted conformational change accompanies occupation of the lacuna. For a series of complexes having R1 = (CH2)n R2 = CH3, and various R3 substituents, the dioxygen complexes have been examined by ESR experiments and simulations. From these, the dependence of the Co-O-O bond angles upon the ligand substituents has been evaluated.