242
Chapter 18 Transition Metal Complexes with CO, N2, NO and O2 Ligands
In 1960, McClure
21 had suggested that this promotion of
2
Fe
to the
intermediate-spin state of configuration (39) occurs when it reacts with O 2 , and we
have given here the valence-bond structure that corresponds to this description.
The results of some Mössbauer measurements
22 indicate that the promotion energy
is small – 250 cm
–1 .
The
2
Fe(II) ( 1) O (
1)
S
S
spin theory for the bonding of O 2 to hemoglobin
has been discussed on a number of occasions
16-21,23-26
. Generalized valence-bond
27
and molecular orbital calculations
28 provide further support for this hypothesis,
although it has been questioned
29 .
Although the possibility also exists that the O 2 may bond symmetrically to the
Fe(II), as occurs in “increased-valence” structures (40) and (41), theoretical and
experimental evidence
27,30 favours the non-symmetrical conformation displayed in
valence-bond structures (35)-(38).
The O-O stretching frequency of 1107
1
cm
for oxyhemoglobin
31 was often
cited
32-34 as evidence that the superoxide anion
2
O
is bonded to low-spin
3
1
2
(
)Fe
S
, with spin-pairing of the unpaired-electrons of these two species to
form S = 0 spin Fe(III) 2
O
linkages. For 2
O
, as in
2
KO , the O-O stretching
frequency is 1145 cm
-1 , whereas that for free ground-state 2
O is 1556 cm
-1 32 .
However, when the O 2 ground-state is bonded to Fe(II), as in structure (38), the
electronegativity of one oxygen atom must be altered relative to that of the other,
and an unequal sharing of the four O-O bonding electrons may therefore occur.
The O-O bond-order for structure (38) can therefore be reduced below the value of
2 for free O 2 , thereby leading to a reduction of the O-O stretching frequency for
oxyhemoglobin
18
. Molecular orbital calculations
28 give an O-O bond-order of 1.6
when the O 2 ground-state is bonded to the Fe(II).
“Increased-valence” descriptions of the bonding for the Fe-CO and Fe-NO
linkages for the CO and NO derivatives of hemoglobin are described in Ref. 17.
18-4(b) Cobalt-Molecular Oxygen Carriers
Molecular oxygen can form both 1:1 and 2:1 adducts (
2
Co O
and
2
Co O Co
)
with different Co(II) complexes. For the 1:1 adducts, the O-O stretching
frequencies are similar to those
32-34 of 2
O
and an unpaired electron is located
mainly on the two oxygen atoms. On the basis of these observations, the adducts
have been formulated
32-34 as
2
Co(III)O
, with the 2
O
forming a coordinate bond
to low-spin Co(III), which has a (3d)
6 configuration. Alternatively, we may formulate
16-20 the electronic structure as
2
Co(II)O , for which the O 2 valence-bond
structure (8) has spin-paired one of its two unpaired electrons with the unpaired
electron of low-spin Co(II) (3d)
7
. Thus, we may generate the “increased-valence”
structure (42) by means of this reaction.
Chapter 18 Transition Metal Complexes with CO, N2, NO and O2 Ligands
In 1960, McClure
21 had suggested that this promotion of
2
Fe
to the
intermediate-spin state of configuration (39) occurs when it reacts with O 2 , and we
have given here the valence-bond structure that corresponds to this description.
The results of some Mössbauer measurements
22 indicate that the promotion energy
is small – 250 cm
–1 .
The
2
Fe(II) ( 1) O (
1)
S
S
spin theory for the bonding of O 2 to hemoglobin
has been discussed on a number of occasions
16-21,23-26
. Generalized valence-bond
27
and molecular orbital calculations
28 provide further support for this hypothesis,
although it has been questioned
29 .
Although the possibility also exists that the O 2 may bond symmetrically to the
Fe(II), as occurs in “increased-valence” structures (40) and (41), theoretical and
experimental evidence
27,30 favours the non-symmetrical conformation displayed in
valence-bond structures (35)-(38).
The O-O stretching frequency of 1107
1
cm
for oxyhemoglobin
31 was often
cited
32-34 as evidence that the superoxide anion
2
O
is bonded to low-spin
3
1
2
(
)Fe
S
, with spin-pairing of the unpaired-electrons of these two species to
form S = 0 spin Fe(III) 2
O
linkages. For 2
O
, as in
2
KO , the O-O stretching
frequency is 1145 cm
-1 , whereas that for free ground-state 2
O is 1556 cm
-1 32 .
However, when the O 2 ground-state is bonded to Fe(II), as in structure (38), the
electronegativity of one oxygen atom must be altered relative to that of the other,
and an unequal sharing of the four O-O bonding electrons may therefore occur.
The O-O bond-order for structure (38) can therefore be reduced below the value of
2 for free O 2 , thereby leading to a reduction of the O-O stretching frequency for
oxyhemoglobin
18
. Molecular orbital calculations
28 give an O-O bond-order of 1.6
when the O 2 ground-state is bonded to the Fe(II).
“Increased-valence” descriptions of the bonding for the Fe-CO and Fe-NO
linkages for the CO and NO derivatives of hemoglobin are described in Ref. 17.
18-4(b) Cobalt-Molecular Oxygen Carriers
Molecular oxygen can form both 1:1 and 2:1 adducts (
2
Co O
and
2
Co O Co
)
with different Co(II) complexes. For the 1:1 adducts, the O-O stretching
frequencies are similar to those
32-34 of 2
O
and an unpaired electron is located
mainly on the two oxygen atoms. On the basis of these observations, the adducts
have been formulated
32-34 as
2
Co(III)O
, with the 2
O
forming a coordinate bond
to low-spin Co(III), which has a (3d)
6 configuration. Alternatively, we may formulate
16-20 the electronic structure as
2
Co(II)O , for which the O 2 valence-bond
structure (8) has spin-paired one of its two unpaired electrons with the unpaired
electron of low-spin Co(II) (3d)
7
. Thus, we may generate the “increased-valence”
structure (42) by means of this reaction.