The molecular structure of Cu(1,5-cod)(hfac) in the gas phase has been determined by electron diffraction, restrained by parameters calculated ab initio (MP2/AE1 level) or using Density Functional Theory (BP86/AE1 level). The most stable structure is one in which one olefinic group of the cyclooctadiene ligand is coordinated to the square-planar copper atom [refined Cu-C distances 194.0(13) and 194.4(9) pm]. The second C=C double bond is weakly associated with the copper atom [Cu...C distances 267.2(23) and 276.9(25) pm], and the cyclooctadiene ligand has a twist-boat conformation, so that the complex has C, symmetry. The nature of the bonding between copper and each of the two olefin moieties has been assessed by topological analysis of the BP86/AE1 total electron density. A form with C symmetry, lying between 2 and 7 kJ mol(-1) above the ground state, is a transition state for exchange of the two olefinic groups. There are also two higher energy conformers, both 10 kJ mol(-1) or more above the ground state. In one of these the cyclooctadiene ligand retains the twist-boat conformation, but the Cu(hfac) moiety is coordinated in the exo position with respect to the noncoordinated olefin, instead of endo, as in the most stable conformer. The molecular symmetry is C-1 in this isomer. In the remaining form the ligand has the chair conformation, and the molecular symmetry is C-s.