398 THE TRANSITION ELEMENTS
OH 2
~^v(^02)
H 20
N — f protein) N—(protein)
Figure 13.4. Schematic representation of haetn (porphin rings not shown)
rim: system—the whole system is a porphin. The sixth position
(Figure 13.4} is occupied either by an oxygen molecule or a water
molecule, and here reversible oxygen uptake can occur, as shown,
thereby enabling oxygen to be transported from one part of the
body to another. Coordination of a ligand CN~ or CO instead of
water prevents this process, and the toxicity of cyanide or carbon
monoxide is, in part due to this fact. :
Low oxidation states
Iron forms the carbonyls Fe(CO) 5 , Fe 2 (CO) 9 and Fe 3 (CO) 12 , In
iron pentacarbonyl. the iron(O) is 5-coordinated, as shown in
Figure 13.5 to give a trigonal bipyramid; the substance is volatile
CO
CO
CO
Figure 13. 5. Structure of iron (0} pentacarbonyl
and covalent. Donation of an electron pair by each CO ligand gives
the iron the configuration of the next noble gas and the ion
[Fe(CO) 4 ]^2 " and some halides Fe(CO) 4 X 2 (X - C Br, I) are known,
the carbonyl halides being octahedral.
THE RUSTING OF IRON
This is the most important reaction of iron from an economic point
of view; essentially, rusting is the formation of hydrated iron(III)
oxide in the presence of oxygen and water. The process is essentially