Usually, you'd expect two compounds with the same composition, atom-to-atom connectivity and symmetry to be chemically identical too. But scientists investigating metal-organic frameworks have discovered a surprising exception to this rule by identifying two isomers with the same symmetry and bonding but different gas storage properties. A team led by Shengqian Ma at the Argonne National Laboratory, Illinois, US, investigated a rod-like tetracarboxylate molecule (ebdc) which can bind to a metal atom from any one of four binding points, one at each corner of a rectangle. When it was heated with a copper salt at 75 °C, a crystal phase formed...