Ascorbic acid, H2L, is a relatively weak bidentate ligand, which coordinates metal ions, Mn+, to form chelates, MHL+n-1 at low and intermediate pH values, and unprotonated chelates, ML+n-2 at high pH. Metal ions capable of undergoing redox reactions catalyze the autoxidation of ascorbic acid through the formation of intermediate metalascorbate-dioxygen complexes. Catalysis of autoxidation by metal chelates seems to occur through the formation of ternary ascorbate complexes of the metal chelates. Ascorbic acid is assigned a significant catalytic role in Udenfriend's system through the formation of an initial ascorbate-Fe(III)-dioxygen complex in which electron transfer to dioxygen results in oxygen activation and oxygen atom insertion.