Bioinorganic chemistry, insofar as it involves catalytic transformations, is largely the chemistry of metalloenzymes (i.e., metalloproteins). Although the complex three-dimensional structure of the protein ligands obviously plays a vital role in determining the activity and stability of metalloenzymes, at the heart of the matter their reactions involve coordination catalysis by metal ions. Thus, at the molecular level we are dealing with homogeneous catalysis in aqueous media.