Structural, Geometrical and Configurational Changes in Metal Complexes of Tetraazamacrocycles with a Pendant Donor Group

Metal complexes of mono-functionalized tetraazamacrocycles show a pH-dependent equilibrium, in which the donor group of the side chain can either bind to the metal ion or not. The structural changes of the metal macrocyclic moiety which take place in this reaction are discussed, and it is shown that the structural problems associated with it are manifold: there are examples in which neither the coordination geometry around the metal ion nor the configuration of the macrocycle are altered; there are cases in which the coordination number of the metal ion is increased, since an additional metal to ligand bond is formed with the side chain donor group, thus giving a new coordination geometry; and finally there is one example in which a drastic rearrangement of the whole metal macrocyclic unit occurs, to give a geometrical and a Configurational change.