A new type of isomerism has been detected in the cyclidene family of lacunar dioxygen carriers, providing an additional Structural variable for the central of their oxygen affinity. In those rare complexes that do not have methyl substituents on the primary macrocycle, NMR and X-ray crystallographic data indicate that, in addition to their usual cis orientation, the bridges can also adopt a trans orientation. In the crystal structure of [Co(C8MeHH[16]cyclidene)](PF6)(2) . 3CH(3)OH, the bridge has this trails orientation with one end in the "lid-on" configuration while the other end is "lid-off’. The trans orientation of the bridge is identified as the principal cause of the decreased dioxygen affinity of such unsubstituted cyclidenes.