Purifying alkenes from alkanes requires cryogenic distillation. This consumes energy equivalent to countries of ca. 5 million people. Replacing distillation with adsorption processes would significantly increase energy efficiency. Trade-offs between kinetics, selectivity, capacity, and heat of adsorption have prevented production of an optimal adsorbent. We report adsorbents that overcome these trade-offs. [Cu-Br]3 and [Cu-H]3 are air-stable trinuclear complexes that undergo reversible solid-state inter-molecular rearrangements to produce dinuclear [Cu-Br⋅(alkene)]2 and [Cu-H⋅(alkene)]2. The reversible solid-state rearrangement, confirmed in situ using powder X-ray diffraction, allows adsorbent design trade-offs to be overcome, coupling low heat of adsorption (−10 to −17 kJ mol−1alkene), high alkene:alkane selectivity (47; 29), and uptake capacity (>2.5 molalkene mol−1Cu3). Most remarkably, [Cu-H]3 displays fast uptake and regenerates capacity within 10 minutes.