Biomedical Engineering Reference
In-Depth Information
10.4 COORDINATION CHEMISTRY
All metal ions are Lewis acids since they can coordinate to free electron pairs (i.e., Lewis bases).
The outcome of this reaction is called a coordination compound or a complex between the central
metal ion (Lewis acid) and the electron donor (Lewis base). A complex is thus composed of ions or
molecules that may exist individually in solution, but in combination they produce the coordination
compound. The ions or molecules coordinated to the central metal ion are called ligands and make
up the coordination sphere. The number of points at which ligands are attached to the metal ion is
called the coordination number. The different categories of ligands are shown in Figure 10.2.
10.4.1 C
HELATE
E
FFECT
Ligands (Lewis bases) with several binding sites are called chelates and form particularly stabile
complexes: metal-chelates. The stability of a coordination compound increases with the number of
binding centers on the ligands (chelate effect). Amino acids, peptides, and proteins contain many
metal binding groups that make them excellent chelates. In proteins, besides peptide NH and C=O
groups many side chains may serve as complex agents for metal ions. These include thiolate in
cysteine, the imidazole ring of histidine, carboxylates of glutamic acid and aspartic acid, and the
amino side chain of lysine.
The rationale behind the chelate effect is quite straightforward. As soon as a metal ion coor-
dinates to one group in a multidentate ligand, the probability for coordination of other potential
donor groups is enhanced. A favorable entropic factor further adds to the stability since chelation is
accompanied by release of nonchelating ligands like water from the coordination sphere.
A closely related effect is termed the macrocyclic effect, which relates to the notion that a com-
plex with a cyclic polydentate ligand has greater thermodynamic stability when compared with a
similar noncyclic ligand. As a consequence, macrocyclic complexes occur widespread in nature,
and are found in, e.g., crown ethers, cryptands (alkali metals), cytochromes (iron), chlorophyll
(magnesium), and coenzyme-B
12
(cobalt).
Ligand
(1) With one or more free
electron pairs
(2) Without free electron pairs but with
π-binding electrons, e.g., ethylene, benzene
(a) No vacant orbitals to
receive electrons from
the metal ion, e.g., H
2
O,
NH
3
, F
-
(b) Vacant orbitals that may
receive π-electrons from the
metal, e.g., CN
-
, aromatic amines
(c) With further π-electrons
that may couple to
vacant metal orbitals, e.g.,
thiolates, phosphines
FIGURE 10.2
Classii cation of the different ligand types.
10.4.2 H
ARD
AND
S
OFT
A
CIDS
AND
B
ASES
(HSAB P
RINCIPLE
)
Metal ions can be divided into two categories:
a. “Class a” metals coordinate to bases that bind strongly to the proton (“hard bases”); i.e.,
bases in the ordinary sense of the word.
b. “Class b” metals bind preferentially to large polarizable or unsaturated bases (“soft bases”)
that usually show insignii cant basicity toward the proton.
