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(3) Tendency to form oligo- and polymers by condensation;
(4) Easy change between high- and low-spin states in ligand fields of medium strength
(spin cross-over; see inset on p. 7, upper part);
(5) Flexibility with respect to the nature of the donating ligand function (see inset on p. 7,
lower part, for ligand classification), the coordination number and coordination
geometry.

Tutorial: Coordination compounds (1): Definition

Coordination compounds, or complexes, are integral molecular or ionic units consisting of a

central metal ion (or atom), bonded to a defined number of ligands in a defined geometrical

arrangement. The ligands can be ions or (induced) dipolar molecules. Each ligand provides

a free electron pair, i.e. the ligands are Lewis bases, while the metal in the coordination

centre is the Lewis acid. The bonding can thus be described in terms of Lewis acid/Lewis

base interaction. Other descriptions of the bonding situation are: (i) donor bond; (ii)

coordinative covalent bond, often denoted by L→M, where L = ligand and M = metal.

Complexes tend to be stable when the overall electron configuration at the metal centre (the

sum of metal valence electrons plus electron pairs provided by the ligands) is 18 (or 16 for

the late transition metals).

q

M + nL ' [ML n] (n = number of ligands, q = charge of the complex)
c(ML )
n
= K
n
c(M) c (L)

-1
K is the stability constant or complex formation constant (pK = -logK); its inverse, K , is

termed dissociation constant.

The average amount of iron in L
the human body (70 kg) is ca. 5 g;
iron is thus the most abundant Haeme-type N F e N
transition metal in our organism. (e.g. cytochromes, haemoglobin) N N
About 70% of this amount is used
for oxygen transport and storage L
(haemoglobin, myoglobin), almost S R F e S F e S R
30% are stored in ferritins (iron Iron-sulfur proteins S R S S R
storage proteins), and about 1% is (e.g. ferredoxins, Rieske proteins)
bound to the transport protein O / N O H O / N
transferrin and to various iron- Two-iron centres F e O F e O
dependent enzymes; cf. the rough (e.g. ribonucleotidereductase) O / N N O O O / N N
classification to the right.

Aqueous iron chemistry

II
2+
3+
The redox potential for the pair Fe /Fe at pH = 7 demonstrates that Fe is easily oxidised to
III
Fe under aerobic conditions (cf. also the tutorial on oxidation and reduction on p 16):
-
2+
3+
Fe ' Fe + e ; E = -0.23 V (at pH 7)
+
-
compare: 2H 2O ' O 2 + 4H + 4e ; E (pH 7) = +0.82 V
-
+
+
+
NADH + H ' NAD + 2H + 2e ; E(pH 7) = -0.32 V
(NADH = nicotine-adenine-dinucleotide in its reduced form)