A terminal, fluxional η4-benzene complex with a thermally accessible triplet state is the primary photoproduct in the intercyclobutadiene haptotropism of (CpCo)phenylenes.

Low-temperature irradiation of linear [3]- and [4]phenylene cyclopentadienylcobalt complexes generates labile, fluxional η4-arene complexes, in which the metal resides on the terminal ring. Warming induces a haptotropic shift to the neighboring cyclobutadiene rings, followed by the previously reported intercyclobutadiene migration. NMR scrutiny of the primary photoproduct reveals a thermally accessible 16-electron cobalt η2-triplet species, which, according to DFT computations, is responsible for the rapid symmetrization of the molecules along their long axes. Calculations indicate that the entire haptotropic manifold along the phenylene frame is governed by dual-state reactivity of alternating 18-electron singlets and 16-electron triplets.