ABSTRACT
The concept of second-sphere coordination, depicted diagrammatically was first put forward by the "founder of coordination chemistry," Alfred Werner. It is evident that ammine ligands provide extremely efficient handles by which crown ethers can be induced into the second coordinationsphere of a transition metal. Since crown ethers form second-sphere adducts with metal complexes bearing NH- and OH-acidic ligands, it seems reasonable to predict that CH-acidic ligands might behave in an analogous manner. Obviously any transition metal complex with a first-sphere coordination ligand bearing acidic hydrogen atoms is a logical candidate for second-sphere coordination by crown ethers. The coordination of a crown ether in the first sphere of a transition metal complex allows the possibility of the crown ether interacting with other first-sphere-coordinated species to act as both a first- and second-sphere ligand. The molecular receptors most closely related to the crown ethers are the ionophore antibiotics.
