METAL ions in the active sites of many metalloenzymes exhibit distinctive spectral and chemical features which are different from those of small inorganic complexes(1,2). These features are the result of the unusual geometric and electronic constraints that are imposed on the metal ion within the protein environment(3). Much effort has been invested to try to mimic this feature of metalloenzymes in synthetic systems, but this remains a formidable task Here we show that one of the key lessons learned from the science of catalytic antibodies-that binding energy can be converted into chemical energy(4)-can be exploited to ’fine-tune’ the physicochemical properties of a metal complex. We show that an antibody’s binding site can reversibly perturb the coordination geometry of a metal ion, and can stabilize a high-energy coordinated species(5). Specifically, antibodies designed to bind the organosilicon compound 1 (Fig, 1) also bind the geometrically similar Cu(I) complex 2. However, the antibody binds a slightly compressed form of 2, which is closer in size to 1. This distortion is manifested by a spectral shift-an ’immunochromic’ effect.