Cation Toxicity and the Stability of Transition-Metal Complexes

Abstract

DESPITE the many important contributions that have been made to the literature during the past century, no completely satisfactory theory of cation toxicity has been formulated. Blake¹ first concluded that there was a correlation between degree of toxicity and atomic weight (hence ‘heavy-metal’ poisons). Matthews² and Jones³ later pointed out a relationship between metal ion toxicity and the ‘solution pressure’ (standard electrode potential) of the corresponding metals. Seifriz⁴ considered various other chemical properties of the ions in his attempt to find a satisfactory chemical explanation for the relative toxicity of cations. Shaw^5–7, basing his arguments on results obtained with enzymes, postulated that metal ions killed aquatic organisms by combining with an essential group (probably—SH) on a key enzyme. Correlation with metal sulphide insolubility was demonstrated. An attempt was also made⁶ to relate the relative inhibitory efficiency of bivalent metal ions toward the enzyme urease with metal complex stability. At that time (early 1953), however, the field of co-ordination chemistry had not reached a stage of sufficient maturity for such a comparison to be successfully made. The explosive growth that has occurred during the past decade in our knowledge of the chemistry of co-ordination compounds necessitates a re-examination of the intriguing possibility that there exists a relationship between metal complex stability and the toxicity of metal ions to living organisms.