Quantum mechanical modeling of the properties of transition metal ions (TMI) in zeolites gives a picture of the material which corresponds to that of a large organometallic system in which the zeolite framework behaves as a multidentate ligand. The electron density is distributed among the whole system with highly delocalized frontier orbitals. Analyses of the electron density changes in Cu-ZSM-5 and Cu-FAU models upon adsorption and desorption of donor or acceptor ligands point to a supermolecular behavior of the whole system where the zeolite framework acts as a reservoir of electronic charge. This molecular description of TMI-zeolites provides a rational explanation of various aspects of their catalytic behavior in the decomposition and selective catalytic reduction (SCR) of nitrogen oxides, such as the nature of the rate determining step and the positive influence of protons in the SCR of NO by NH3. (C) 2003 Elsevier Science (USA). All rights reserved.