Stabilization of unusual oxidation states by coordination

Low oxidation states are stabilized by ligands which can accept electrons from the metal (through r-bonding) and in this way increase the positive charge on the metal. These ligands are called k acids. The ligands NO and CO are very effective in this regard CN, bipy, phosphines, and arsines are also effective. Examples of compounds in which the metal has an unusually low oxidation state are Na[V(CO)6], [Cr(bipy)s], and K4[Ni(CN)4]. 

The electrode potentials for metal ions change markedly as the type of ligand is changed (Table 5.2). As water is replaced by CN , EDTA, or NH3 in complexes of Fe and Co, the tendency for oxidation to the state is 

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