Molecular shape recognition complexation occurs when a small guest molecule (of distinct shape) fits into the molecular sized cavity or channel of larger host molecule. Many inorganic, organic and bio-organic host-guest complexes are known. Such complexes are interesting because the chemical environment inside the cavity is usually very different form outside. Interesting chemistry is often observed.
For more information look in the Chemogenesis
webbook sections on Lewis
acids and Lewis bases and the Lewis
acid/base interaction matrix.