Chemistry Reference
In-Depth Information
immediately formed as the metal ion is hydrated, a process which simply involves a set of
water molecules rapidly binding to the metal ion as ligands. An energy change is associated
with this process - the heat of hydration. If the solvent is removed by evaporation and the
residual solid gently dried, a blue solid is recovered. This is the hydrated, or complexed,
salt that has the formulation CuSO
4
·
5H
2
O. That the water molecules are tightly bound to
the copper ion can be shown by simply measuring weight change as temperature is slowly
raised. What is observed is that all water is not removed simply by heating to 100
◦
C, but
is eventually removed fully only following heating to over 200
◦
C for an extended period,
with recovery after that stage of the anhydrous species. Application of an array of advanced
experimental methods allows us to observe the species in solution also; not only can we
observe the presence of separate cations and anions, but the size, shape and environment
of the ions can be elucidated. This confirms that the copper ion exists with a well-defined
sheath of water molecules, the inner coordination sphere, which are in effect simple ligands,
each water molecule attached to the central metal through a coordinate covalent bond via
an oxygen lone pair. When this entity is ionic, as is the case for copper(II), this complex
is surrounded by a partially ordered
outer
(or
secondary
)
coordination sphere
where water
molecules are hydrogen-bonded to the inner-sphere ligated water molecules; a third and
subsequent sheath surrounds the second layer, the process continuing until the layers be-
come indistinguishable from the bulk water. The various layers moving outwards from the
centre undergo successively decreasing compression as a result, in the simplest view, of the
progressively diminishing electrostatic influence of the metal ion.
It is also important to think about the lifetime of a particular complex ion. For an aquated
metal ion in pure water, there is but one ligand type available. However, it is not correct
to assume that, once formed, a complex ion inevitably remains with the same set of water
molecules for ever. In solution, it is possible (indeed usual) for water molecules in the
outer coordination sphere to change places with water molecules in the inner coordination
sphere. Obviously, this is a difficult process to observe, since there has been no real change
in the metal environment when one water molecule replaces another - a little like taking a
cold can out of a refrigerator and replacing it with another warm one of the same kind, so
that no one can tell unless they pick up the warm can. At the molecular level, one can adopt
the cold/warm can concept to probe what is called ligand exchange, by adding water with
a different oxygen isotope present and following its uptake into the coordination sphere.
The facility of this water exchange process varies significantly with the type and oxidation
state of the metal ion. Moreover, the rate of exchange varies not only with metal but with
ligand - to the point where longevity of a particular complex can indeed be extreme, or the
coordination sphere is for all intents and purposes fixed. We shall return to the concept of
ligand exchange again in Chapter 5.
2.1.3
Putting the Bite on Metals - Chelation
The classic simple ligand is ammonia, since it offers but one lone pair of electrons, and thus
cannot form more than one coordinate covalent bond (Figure 2.2). A water molecule has
two lone pairs of electrons on the oxygen, yet also usually forms one coordinate covalent
bond. If one looks at the arrangement of lone pairs, this is hardly surprising; once one
coordinate bond is formed, the remaining lone pair points in the wrong direction to allow
it to become attached to the same metal ion - only through attachment to a different metal
could this lone pair achieve coordination (a situation for the ligand called
bridging
). We
shall return to examine whether this can actually happen for a water molecule later.
