An attractive strategy for achieving solar-thermal energy conversion is to harvest sunlight in the form of activated chemical bonds through photoisomerization of a suitable molecule that can release, on demand, such stored and transportable energy by thermal reversal to its original form.[1] When such reversible isomerizations entail significant topological alterations, they provide blueprints for eliciting further functions, for example in switches, machines, datastorage, sensors, and other devices. [1,2] Because of their expanded tunability and generally favourable electronic absorption regimes, organometallic complexes are advantageous in this respect, yet have remained relatively unexplored.[3] Among them, topologically simplest are metallohaptotropic arrays in which a single metal moiety photomigrates, thermally reversibly, to a higher-energy position along a fused p framework, without the assistance of additives.[4] Only two such systems are known, [Mo(PMe 3 ) 3 ] complexes of indole and quinazoline, discovered as part of a study focusing on catalytic hydrogenations of heterocycles. [5] We report 1) the photothermal reversibility of {CpCo} complexes of linear phenylenes [6] by a novel mode of haptotropism, namely, h 4 :h 4 from one cyclobutadiene ring to another (Scheme 1); 2) the first X-ray structures of metalated linear phenylenes, illustrating the aromatization of the ligand on complexation; 3) mechanistic aspects of the isomerization cycles; and 4) a DFT study providing a detailed picture of how the {CpCo} unit moves across the arene separating the two cyclobutadiene rings.The new complexes, 2, 5, and 6-8, were made from the respective 2,3,6,9-tetraethynyl linear [3]phenylene, 1,2,4,5-tetraethynylbenzene, and 2,3-diethynylbiphenylene ([D 2 ] for 8) synthons and their cocyclization with bis(trimethylsilyl)-acetylene (BTMSA) in the presence of stoichiometric [CpCo(CO) 2 ] (MeCp for 7), a strategy employed previously Scheme 1. Photo-thermal cycles of CpCo-phenylene complexes.