Abstract
Inversions of the usual stability constant order in which a subsequent ligand binds more strongly than a former one are described in a model in which the coordination number of the metal ion decreases. Decreasing the coordination number shortens and strengthens the metal-ligand bond. Inversions of the usual bonding order are often termed cooperative, and the model is patterned after the allosteric model used to describe cooperative oxygen binding in hemoglobin. The model is developed and applied to examples that include Al(III), Ni(II), Zn(II), Cu(I), Ag(I), and Hg(II).