22-3
Reactions of O2 with Fe(II) Porphyrin Complexes
287
The “increased-valence” formulation of the reaction steps for
3
2
2
NO O
NO O



involves the structures (11)-(15),
The delocalization of a nitrogen lone-pair electron of structure (13) into an N-Ο
bonding orbital has two effects. It assists with the weakening of the adjacent
—
  bond, and generates a fractional unpaired-electron charge on the nitrogen
atom. The results of electron spin resonance measurements
5 show that such a
charge is present in the free NO 2 molecule. The “increased-valence” structure of
(15) for NO 2 has been described previously in section 11-8.
22-3 Reactions of O 2 with Fe(II) Porphyrin Complexes
In Section 18-4, we have provided an “increased-valence” description of the bonding of O 2 to the Fe(II) porphyrin complex, hemoglobin. For each
2
Fe(II)O linkage
of the ground-state of oxyhemoglobin, the O 2 ground-state is bonded to the
intermediate-spin Fe(II) in the “increased-valence” structure. A number of other
Fe(II) porphyrin complexes are irreversibly oxidized by O 2 to form oxo-bridged
dimers
6
. One reaction scheme that has been proposed
6 is that of (e).
2
2
2
Fe(II) O
Fe(II)O
(Fe(III)O )

 
Fe(II)O 2 + Fe(II)  Fe(II)O 2 Fe(II)
(Fe(III)O 2
2- Fe(III))
2
Fe(II)O Fe(II) 2Fe(II)O
(Fe(III)O )


(e)
2
Fe(II)O Fe(II) Fe(II)OFe(II)
(Fe(III)O Fe(III))


