Stereochemical Change

The kinetic and mechanistic aspects of this general area tend to be strongly dependent on the particular system. This makes general treatments and explanations impossible, at least at the current stage of understanding. Various aspects of this area have been summarized in some general reviews. Ligands bonded to a metal can undergo a number of structural changes that do not involve complete breaking of the metal-ligand bond(s). Such processes are the subject of the following sections. Many chelate ligands have conformers that can interconvert. For example, the conformers of ethylenediamine interchange by rotation about the carbon-carbon bond, as shown in the following structures: The Ha and Ha' protons are magnetically different from the Hb and Hb' protons, so their interconversion can, in principle, be studied by NMR. These protons may be referred to as exo and endo, respectively. In simple systems, their interconversion is too rapid (k >106 s-1) for this method. However, if there is some constraint (e.g., CH3 groups) or if the coordinating atoms are part of a larger chelate system, then interconversion is slow enough to be detected by NMR. In nonplanar Fe(III)- tetraphenylporphyrinates, the ring inversion rates vary widely, depending on the axial ligand and the substituents on the porphyrin.