text: The rates at which ligands exchange between the first and the second coordination sphere is the first step in ligand substitution reactions. In associative ligand substitution, the entering nucleophile resides in the second coordination sphere. These effects are relevant to practical applications such as contrast agents used in MRI.
  The energetics of inner sphere electron transfer reactions are discussed in terms of second coordination sphere.  Some proton coupled electron transfer reactions involve atom transfer between the second coordination spheres of the reactants:
  :[Fe*(HO)]  +  [Fe(HO)(OH)]  →   [Fe(HO)]  +  [Fe*(HO)(OH)]