Cooperative binding, whereby an initial binding event facilitates the uptake of additional substrate molecules, is common in biological systems such as haemoglobin1,2. It was recently shown that porous solids that exhibit cooperative binding have substantial energetic benefits over traditional adsorbents3, but few guidelines currently exist for the design of such materials. In principle, metal-organic frameworks that contain coordinatively unsaturated metal centres could act as both selective4,5,6,7and cooperative adsorbents if guest binding at one site were to trigger an electronic transformation that subsequently altered the binding properties at neighbouring metal sites8,9,10. Here we illustrate this concept through the selective adsorption of carbon monoxide (CO) in a series of metal-organic frameworks featuring coordinatively unsaturated iron(II) sites. Functioning via a mechanism by which neighbouring iron(II) sites undergo a spin-state transition above a threshold CO pressure, these materials exhibit large CO separation capacities with only small changes in temperature. The very low regeneration energies that result may enable more efficient Fischer-Tropsch conversions and extraction of CO from industrial waste feeds, which currently underutilize this versatile carbon synthon11. The electronic basis for the cooperative adsorption demonstrated here could provide a general strategy for designing efficient and selective adsorbents suitable for various separations.