Agriculture Reference
In-Depth Information
Lewis base and the metal as a Lewis acid. A characteristic of ligands is that they
have a lone pair of electrons which they can donate to empty electron orbitals
on the metal. Some ligands also have empty p- or d-orbitals and can produce
complexes in which a double bond is formed with the metal: a sigma bond by
donation of the lone pair from the ligand to the metal and a pi bond by back
donation of electrons on the metal to empty d-orbitals on the ligand. The term
chelate is reserved for species involving polydentate ligands that form a ring of
atoms including the metal.
Inorganic and organic ligands contain the following electron donor atoms from
Groups IVB to VIIB of the Periodic Table:
CNOF
PS l
As Se Br
Te
I
Formation of coordination complexes is typical of transition metals, but other
metals also form complexes. The tendency to form complexes is a function of
the metal's electron configuration and the nature of its outer electron orbitals.
Metal cations can be classified into types A and B based on their coordination
characteristics, as shown in Table 3.5. A-type cations, which tend to be from the
left side of the Periodic Table, have the inert-gas type electron configuration with
largely empty d-orbitals. They can be imagined as having electron sheaths not
easily deformed under the influence of the electric fields around neighbouring
ions. B-type cations have a more readily deformable electron sheath.
In consequence, A-type cations form complexes preferentially with the fluoride
ion and ligands having oxygen as their electron donor atom. They are attracted to
H
2
O more strongly than to NH
3
or CN
−
, and they do not form sulfides because
OH
−
ions readily displace HS
−
or S
2
−
ions. They tend to form sparingly soluble
precipitates with OH
−
,
CO
3
2
−
and PO
4
3
−
. By contrast, B-type cations coordinate
preferentially with ligands containing I, S or N as electron donors. They may bind
NH
3
more strongly than H
2
OandCN
−
more strongly than OH
−
, and they tend
not to form complexes with the main functional groups in organic matter, which
have O as electron donor. They form insoluble sulfides and soluble complexes
with S
2
−
and HS
−
.
Table 3.6 shows the major inorganic species expected in a solution with a
composition typical of natural fresh water. Some calculations for organic ligands
in submerged soil solutions are given in Section 3.7.
3.1.3 EQUILIBRIUM CALCULATIONS
Complete calculations of chemical equilibria in natural waters and soil solutions
are complicated because such a large number of solutes, solids and gases are
