Additional cyanide ions may bond to the iron to form Fe(CN)2, Fe(CN)3-, Fe(CN)42-,
Fe(CN)53-, and Fe(CN)64-, where the water molecules still bound to the iron(II) are
omitted for simplicity. This phenomenon is called complexation; the species that
binds with the metal ion, CN- in the example above, is called a ligand, and the
product in which the ligand is bound with the metal ion is a complex, complex ion,
or coordination compound. A special case of complexation in which a ligand bonds
in two or more places to a metal ion is called chelation. In addition to being present
as metal complexes, metals may occur in water as organometallic compounds containing
carbon-to-metal bonds. The solubilities, transport properties, and biological
effects of such species are often vastly different from those of the metal ions themselves.
Subsequent sections of this chapter consider metal species with an emphasis
upon metal complexation, especially chelation, in which particularly strong metal
complexes are formed.In the example above, the cyanide ion is a unidentate ligand, which means that
it possesses only one site that bonds to a metal ion. Complexes of unidentate ligands
are of relatively little importance in solution in natural waters. Of considerably more
importance are complexes with chelating agents. A chelating agent has more than
one atom that may be bonded to a central metal ion at one time to form a ring
structure. Thus, pyrophosphate ion, P2O74-, bonds to two sites on a calcium ion to
form a chelate:
Additional cyanide ions may bond to the iron to form Fe(CN)2, Fe(CN)3-, Fe(CN)42-,Fe(CN)53-, and Fe(CN)64-, where the water molecules still bound to the iron(II) areomitted for simplicity. This phenomenon is called complexation; the species thatbinds with the metal ion, CN- in the example above, is called a ligand, and theproduct in which the ligand is bound with the metal ion is a complex, complex ion,or coordination compound. A special case of complexation in which a ligand bondsin two or more places to a metal ion is called chelation. In addition to being presentas metal complexes, metals may occur in water as organometallic compounds containingcarbon-to-metal bonds. The solubilities, transport properties, and biologicaleffects of such species are often vastly different from those of the metal ions themselves.Subsequent sections of this chapter consider metal species with an emphasisupon metal complexation, especially chelation, in which particularly strong metalcomplexes are formed.In the example above, the cyanide ion is a unidentate ligand, which means thatit possesses only one site that bonds to a metal ion. Complexes of unidentate ligandsare of relatively little importance in solution in natural waters. Of considerably moreimportance are complexes with chelating agents. A chelating agent has more thanone atom that may be bonded to a central metal ion at one time to form a ringstructure. Thus, pyrophosphate ion, P2O74-, bonds to two sites on a calcium ion toform a chelate:
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