Biomedical Engineering Reference
In-Depth Information
e
3
Δ 0
5
Δ 0
2
Δ 0
5
t 2
FIGURE 10.3 Crystal splitting of d -orbitals. The diagram shows the splitting of a set of d -orbitals in a metal
ion complex having an octahedral symmetry. The energy difference between e and t 2 orbitals is designated Δ 0 .
Δ 0 , is called the
ligand i eld splitting. In a tetrahedral coordination compound the arrangement is the opposite.
By preferentially i lling up the lower-lying t 2 -orbitals the d electrons will stabilize the system
relative to an average arrangement of the electrons among all available orbitals. The gain in binding
energy obtained by distributing the charges in a nonsymmetrical way is called crystal i eld stabiliza-
tion energy (CFSE). The e -orbitals clearly have higher energy than the t 2 -orbitals. We now assign
an energy of −2/5 ×
three lower-lying orbitals are of the t 2 type (cf. Figure 10.3). The energy difference,
Δ 0 to the e -orbitals, and can calculate the
stabilization energies for complexes with any number of d electrons. For example, a d 5 high-spin
octahedral complex will acquire a CFSE of (−3 × 2/5 + 2 × 3/5) ×
Δ 0 to the three t 2 -orbitals and +3/5 ×
Δ 0 equal to 0. In a low-spin d 5
complex, the energy will be lowered by −5 × 2/5 ×
Δ 0 or −2 ×
Δ 0 . Thus, the latter will be considerably
less reactive than the former.
The d 8 coni guration deserves special attention since this system leads to very stable and inert
square planar compounds. Platinum(II) complexes belong to this group and will be discussed in
detail in Section 10.6.4. Cu(II) ( d 9 ) and Zn(II) ( d 10 ) coordination compounds are found frequently
in enzyme systems where their large reactivity is fully utilized.
10.4.4 R EDOX R EACTIONS
A reduction-oxidation (redox) reaction is a process in which changes in oxidation states or oxidation
numbers take place. Many transition metals exist in several stable oxidation states, which render them
particularly interesting also in biological redox chemistry. Redox reactions play a central role in
biochemistry; pertinent examples are photosynthesis and respiration where cascades of electron transfer
reactions are coupled to synthesis of high-energy molecules like ATP and similar compounds. However,
one of the expenses for living under oxygen rich conditions is the danger of unwanted radical formations.
Oxygen easily gets reduced to hydrogen peroxide, and in the presence of reducing metal ions like Fe 2+
or Cu + further reactions may take place like the Fenton reaction, generating hydroxyl radicals:
_
« FeO + + . OH
Fe 2+ + HO 2
Fortunately, the organism possesses a number of effective chelates, proteins like albumin, transferrin,
and the like that, to a certain limit, will sequester redox-active iron- and copper ions.
10.5 CHELATE THERAPY
Heavy metals pose health hazards and toxication can be treated by using antagonists (chelate
therapy), which involves complex binding (sequestration) and transport of acutely poisonous elements
by means of polydentate ligands (Table 10.4). Obviously, selectivity plays a vital role and thus
 
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