ABSTRACT
A review of the general coordination chemistry of metal ions is usually included in textbooks on the biological chemistry of metals (e.g., da Silva and Williams, 1991) or handbooks on their toxicology (e.g., Vouk, 1986; Martin, 1988). It is the goal of this article to take a more comprehensive perspective since some properties in addition to those displayed in aqueous media are relevant to toxic responses. This broader approach is consistent with the definition of speciation of the elements formulated in 1991 and embraced in 1994, respectively, at the First and Second International Symposium on Speciation of Elements in Toxicology and in Environmental and Biological Sciences held at Loen, Norway (Nieboer, 1992a; Nieboer and Thomassen, 1995):
Speciation is the occurrence of an element in separate, identifiable forms (i.e., chemical, physical or morphological state).
