A major advance has been made in the incremental molecular design of long-lived cobalt(II) dioxygen carriers. Preceding mechanistic studies revealed that ionizable methyl groups trigger the autoxidation of the O-2 adduct of the cobalt(rr) cyclidene. Two members of a new family of unsubstituted (no methyl groups) lacunar cyclidene dioxygen carriers have been prepared in an eight-step synthesis, and a complex with a hexamethylene bridge has been structurally characterized. in contrast to previously studied cyclidenes, these materials bear no substituents on the chelated macrocyclic platform. As anticipated, the rates of autoxidation of these unsubstituted cyclidene complexes were found to be 5-8 times slower than those for the most stable previously known cyclidene derivatives. Because of the absence of Me-Me vicinal repulsion, the CG bridge assumes a zig-zag conformation directly across the cavity. The accompanying, relatively low, dioxygen affinity is explained on the basis of electronic and steric factors. The rates of dioxygen binding to these newly prepared cobalt(II) unsubstituted cyclidenes are fast and approximately equal to the corresponding values for their Me-substituted analogs. Consequently, differences in dissociation rates are responsible for the differences in O-2 affinities. This isa clear example of an unusual steric effect for O-2 adducts.