Chloride Control of a Catalyst: Allosteric regulation of proteins and enzymes allows their conformation to be controlled by changes in the concentration of effector-binding molecules; reactivity can be controlled by changing access to the active catalytic site. The supramolecular control of conformation through multiple weaker interactions has been exploited in organometallic catalysts that can open or close access to reactive sites around a metal center. Yoon et al. (p. 66 ) now show how chloride ions can be used to control a rhodium polymerization catalyst in which two metal centers are bridged by a linker bearing aromatic groups. When Cl ⁻ is present, reactive sites remain available on the metal centers. When Cl ⁻ is removed by abstracting agents, aromatic groups on one of the other ligands on the Rh centers can bind to the linker to form a triple-layer sandwich structure. Formation of this supramolecular structure blocks access of reactants to the metal centers.