Chemistry Reference
In-Depth Information
2
Electronic Structure
of Metals and Atomic
Parameters
2.1 ABOUT QSAR AND THE DESCRIPTORS
OF CHEMICAL STRUCTURE
In the absence of precise information regarding the biological mechanisms that
contribute to the production of the biological activity, and in the absence of satisfac-
tory information regarding the state of the metal ions in biological environments (this
could be either the free ion state or, most probably, the bonded form in a chemical
compound), a suitable quantitative structure-activity relationship (QSAR) approach
would be to choose descriptors that represent the long path that the metal ions have
to travel up to the biological target (Schwietert and McCue 1999). The choice of
descriptors must consider all the physical-chemical processes that happen to the
metal ion or metal compound up to the last event, which is the interaction with the
biological receptor. Correspondingly, a good correlation of metal biological activ-
ity with descriptors depends on a better understanding of the physicochemical or
biological mechanisms in which metal ions participate in biological environments.
Many descriptors that can be used for metals are generally common to chemi-
cal structures—for example, molar refractivity, electronegativity, and so on. Other
descriptors are specific for metals or metal ions, depending on their electronic con-
figuration and their position in the periodic table.
A review of the parameters found in the literature to correlate with metal toxicity shows
the use of atomic parameters such as atomic radii and the ionization energies; the use of
basic chemical properties such as the valence; the electrochemical character and thermo-
dynamic measures that describe the electrochemical reactions of the metals; the theory
of hard and soft acids and bases; and properties of the metal compounds, for example, the
solubility of metal compounds or other particularities of the metal compounds (a review
is found in Walker et al.,
2003). To better understand such descriptors, some important
characteristics of metals are discussed in this and the following chapter. In addition
to the descriptors relevant for the free metal ions, the array of structural descriptors is
greatly enlarged by the use of the descriptors for organometallic complexes (Chapter 4).
2.2 GENERAL PROPERTIES OF METALS
More than 80 chemical elements in the periodic table are classified as metals (~65%):
they all have a set of physical and chemical properties that characterize the metallic
character. Another 18 elements are classified as nonmetals, while 7 elements that
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