Metals in the Molecular Environment

Metals can be classified based on an array of qualities. Some are more useful than others for quantitatively predicting intermetal differences in bioactivity. Following the lead of numerous authors, most notably Nieboer and Richardson (1980), this 

The primary purpose of classifying [metal ions] in (a), or hard, and (b) or soft, is to correlate a large mass of experimental facts. All the criteria used for the classification are thus purely empirical they simply express the very different chemical behavior of various [metal ions]. 

Classification of Metal Ions According to Nieboer and Richardson (1980) 

Note The actinides and lanthanides are class (a) metals. Although placed in this table as an intermediate metal ion, Pb tends toward class (b) more than most intermediate metal divalent ions in that part of this table. Cd also is classified as being along the line between class (b) and intermediate metal ions. 

The class (a) and (b) metals are clustered predictably in the periodic table, with intermediate (borderline) metals being found between these clusters. The exact borders for these classes of metals vary in the published literature because the tendencies used to separate the metals are continuous and a discrete classification is partially arbitrary. 

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