Chemistry Reference
In-Depth Information
2.4.3
Supersized - Binding to Macromolecules
There is in principle no limit to the size of a molecule that may bind metal ions. What we
often see is that as molecules get larger they become less soluble, which places a limit on
their capacity to coordinate to metal ions in solution. However, what we also now know
is that even solids carrying groups capable of coordinating to metal ions can adsorb metal
ions from a solution with which they are in contact onto the solid surface, effectively
removing them from solution through complexation (a process called
chemisorption
). Thus
complexation is not restricted to the liquid state, and will occur in the solid state; as
discussed earlier, it also can occur in the gas phase.
Large, supersized molecules (usually termed
polymers
) are common. Many biomolecules
are composed of complex polymeric units (peptides or nucleotides), and many have the
potential to bind metal ions; in fact, the metal ions may contribute to the shape and chemical
activity of the polymer. So size is no object. What remains true even in these huge molecules
is that the rules of coordination do not change, and any particular type of metal ion tends
to demand the same type of coordination environment or number of donors irrespective of
the size of the ligand. This means that, if a metal ion prefers to form six bonds to six donor
groups, it will usually achieve this whether only one donor group is offered by a molecule
(by binding six separate molecules) or whether one hundred donor groups are offered by a
molecule (by binding selectively only six of the one hundred donors available).
2.5
A Separate Race - Organometallic Species
We have mentioned earlier the prospect of the carbanion H
3
C
−
being an effective ligand,
since it offers an electron pair donor in the same way that ammonia does. To emphasize
this aspect, even simple compounds like [M(CH
3
)(NH
3
)
5
]
n
+
have been prepared in recent
decades. A vast area of chemistry has grown featuring metal-carbon bonds called, not
surprisingly, organometallic chemistry. Typical ligands met in this area are carbon monox-
ide, alkenes and arenes; it is also conventional practice to include hydride and phosphine
ligands. There has been a tendency to regard this field as somehow separate from traditional
compounds, but the division is somewhat artificial, although we will see there are some
good reasons for this schism. One simple distinction is that, unlike the usually ionic Werner-
style compounds, many organometallic compounds are neutral, low melting and boiling
point compounds that dissolve in organic solvents, and display greater ligand reactivity.
Further, they usually feature metals in low oxidation states, and in polymetallic systems are
more likely to involve direct metal-metal bonds. They can also display ligand types and
structural characteristics that were not anticipated from classical Werner-style coordination
chemistry, and that require more sophisticated models for satisfactory interpretation.
One distinction that we meet relates to the mode of bonding in organometallic com-
pounds. We have seen how H
3
C
−
can make use of its lone pair in the conventional manner
to coordinate, forming a
covalent bond. This approach is less obvious for a molecule
like ethene (H
2
C CH
2
), which has been shown to usually involve side-on bonding with
the two carbon centres equidistant from the metal centre. This is not an arrangement easily
dealt with by the simple covalent bonding model, which can accommodate bonding only
by regarding the molecule as formally H
2
C
− +
CH
2
with the carbanion carrying the lone
pair alone able to bond covalently. There are clearly problems with this approach, since
the bonding arrangement in the resulting complex involves a single
bond, leaving the
