Chemistry Reference
In-Depth Information
long periods, would suggest that free active metals would increasingly end up dissolved
as their ions in the ocean, and this is clearly not so - most metal ion concentrations in the
ocean are very low (apart from alkali metal ions, being
0.001 ppm). In reality, because
reactive elemental-state metals are made mainly through human action, the contribution to
the biosphere even by reversion to ionic forms will be small. Most metals are locked up as
ions in rocks - particularly as highly insoluble oxides, sulfides, sulfates or carbonates that
will dissolve only with human interception, through reaction with strong acids or ligands.
Even if they enter the biosphere as soluble complex ions, they are prone to chemistry that
leads to re-precipitation. The classic example is dissolved iron(II), which readily undergoes
aerial oxidation to Fe(III) and precipitation as a hydroxide, followed by dehydration to an
oxide, all occurring below neutral pH.
Thus in the laboratory we tend to meet almost all metals in a pure form as synthetic
cationic salts of common anions. These tend to be halides or sulfates, and it is these metal
salts, hydrated or anhydrous, that form the entry point to almost all of metal coordination
chemistry. In nature, it is no accident that metal ions that are relatively common tend to find
roles, mediated of course by their chemical and electrochemical properties. Thus iron is
heavily used not only because it is common, but also because it forms strong complexes with
available biomolecules and has an Fe(II)/(III) redox couple that is accessible by biological
oxidants and reductants and thus useful to drive some biochemical processes.
1.3.1
Metals in the Natural World
Most metals in the Earth's crust are located in highly inorganic environments - as compo-
nents of rocks or soils on land or under water. Where metals are aggregated in local high
concentrations through geological processes, these may be sufficient in amount and con-
centration to represent an ore deposit, which is really an economic rather than a scientific
definition. In addition, metals are present in water bodies as dissolved cations; their concen-
trations can be in a very few cases substantial, as is the case with sodium ion in seawater.
However, even if present in very low concentration, as for gold in seawater, the size of the
oceans means that there is a substantial amount of gold (and other metals) dispersed in the
aquatic environment. The other location of metals is within living organisms, where, of the
transition metals, iron, zinc and copper predominate. On rare occasions the concentration
of another metal may be relatively high; this is the case in some plants that tolerate and con-
centrate particular metal ions, such as nickel in
Hybanthus floribundus
, native to Western
Australia, which can be hyper-accumulated up to
50 mg per gram dry weight. Levels of
metal ions in animals and in particular plants vary with species and environment. However,
generally metals are present in nature in only trace amounts (Table 1.1). High levels of
most metal ions are toxic to living species; for example ryegrass displays a toxicity order
Cu
∼
Ni
Mn
Pb
Cd
Zn
Al
Hg
Cr
Fe, with each species displaying a
unique trend.
Metals were eventually recognized as having a presence in a range of biomolecules.
Where metal cations appear in living things, their presence is rarely if ever simply fortuitous.
Rather, they play a particular role, from simply providing an ionic environment through to
being at the key active site for reactions in a large enzyme. Notably, it is the lighter alkali,
alkaline earth and transition elements that dominate the metals present in living organisms.
Of transition metals, although iron, copper and zinc are most dominant, almost all of the
first row transition elements play some part in the functioning of organisms. Nevertheless,
even heavier elements such as molybdenum and tungsten are found to have some roles.
