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II
Pt : Chemotherapy (e.g. with cisplatin cis-[Pt(NH 3) 2Cl 2]) of cancer (ovaria, testes)

I
Au : Therapy of rheumatic arthritis.

III
Sb : Treatment of inflammatory skin pimples like acne.

III
Bi : Treatment of gastritis.

Transition metal ions commonly are not present in a free form, but rather coordinated
(complexed) to ligands. In particular, this applies to metal ions in the active centres of enzymes
or otherwise integrated into peptides and proteins. Examples for the respective ligands are
listed below (N-functional: peptide moiety, porphinogenes, histidine; O-functional: tyrosinate,
serinate, glutamate and aspartate; S-functional: cysteinate and methionine):

O O
H -H + H H O
-C N C -C N C -C N C N
R R H H N N
ε R N
NH +
R = -CH 2 -CH 2 N -H -CH 2 N
δ N NH N

O
CH 2 O - Tyr (Y) -(CH ) C n = 1: Asp (D); n = 2: Glu (E)
2 n
O -
O O O M
C C M C
O M O O M
end-on side-on
monodentate bidentate (chelate) end-on bridging

- CH 3
-
O
CH CH S - CH Se CH S
2
2
2
2
Ser (S) Cys (C) Selenocysteinate Met (M)

Additional inorganic ligands:
H 2
O O H O O-O O-O S 2
H
Aqua Hydroxido Oxido [Oxo] Peroxido Hyperoxido Sulfido [Thio]

2. Iron

Iron takes over a central role in biological events (see also its role in the primordial synthesis of
organic compounds; ch. 1). On the one hand, this is due to its general availability (iron is
abundant and ubiquitous in the geo- and biospheres), on the other hand, iron has specific and
“biologically suitable” properties otherwise not (or less) available with other transition metals:
(1) Ease of change between the oxidation states +II and +III (and disposability also of the
oxidation states +IV and +V);
(2) Formation of hexaaqua cations in water; these hexaaqua cations are Brønsted acids;