Reactions Involving the Metal Oxidation State

Because many complexed metals ions have a range of oxidation states accessible in usual solvents, it is not surprising to find that syntheses may involve oxidation-reduction reactions. The classical example is cobalt, which is usually supplied commercially as Co(II) salts, but whose complexes are best known as Co(in) compounds. This is because Co(ni) compounds are inert and usually robust, readily isolable compounds, whereas Co(II) 

Substitution reactions without redox chemistry being involved are available for Co(III), but not commonly met. One example is the use of [Com(C03)3]3- as a synthon, since the chelated carbonate ion is readily displaced by other better chelating ligands such as polyamines. An example of this type of reaction, where the entering polyamine (cyclam) is a saturated and flexible macrocyclic tetraamine, is (6.27). 

Many other metal complexes can be chemically oxidized to higher oxidation states with an appropriate oxidizing agent. This can occur with complete preservation of the coordination sphere (6.29), which means the oxidation-reduction reaction is reversible if a suitable reducing agent is then employed for reduction of the oxidized form. 

Alternatively, the coordination number and/or the ligand set can change substantially in an irreversible oxidation reaction. Any reduction reaction of the product will then not regenerate the original complex. 

Not only are oxidation reactions fairly common, but also one may employ reduction reactions in simple synthetic paths, provided the reduced form is also in a stable oxidation state. Two examples of reduction reactions, with the metal oxidation states included, are given in (6.30) and (6.31). 

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