1963 Volume 36 Issue 6 Pages 669-675
The stability constants of 4-carboxy- and 4-sulfocatechol chelates with bivalent transition metals and alkaline earth metals have been evaluated in aqueous media at 30.0±0.1°C and 0.10 M of ionic strength. The stability orders, manganese<cobalt∼nickel<zinc<copper for transition metal chelates and strontium<calcium<magnesium for alkaline earth metal chelates, have been found to hold in these catechol chelate systems. A unique stability sequence, nickel<zinc, has been discussed in terms of the electron delocalization effect, in which metal d orbitals, lone pair orbitals of catechol oxygens and π orbitals of the benzene ring are involved. A reverse relationship for ligand basicity and chelate stability was observed for copper(II) and zinc(II) chelates, and the stability of metal chelates was found to increase in the sequence; catechol<4-carboxycatechol<catechol-4-sulfonate<Tiron. Substituent groups were considered to play an important role in this case.
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