angles, as recently computed by Hammani et al. [11] using the coupled electron pair
approximation (CEPA) method with a correlation consistent basis set, are,
respectively, equal to 174.3° and 117.6°, while the O–C, C–OH, and O–H bond
lengths are, respectively, of 112.4, 122.9, and 98.4 pm. Theoretical investigations
agree on the fact that the OCO backbone of this molecule is slightly bent, with a
trans configuration and the C–O and C–OH bond distances being, respectively,
shorter and longer than what is observed in the non-protonated molecule [6b, c, 11].
The positive charge is mainly localized on the carbon and hydrogen atoms with the
following repartition: C, +0.584; H, +0.350; O, +0.012; and (H)O, +0.054. The
alternative C-protonation affords a cyclic dioxyril cation with a very weak O–O
(1.733 Å) bond and a very high positive charge (+0.89) at the central CH unit.
Alternative to the protonation is the interaction with metal cations: several
examples of O-coordinated CO 2 to metal centers are known. The available structural data show unambiguously that, whenever CO 2 behaves exclusively as an
O-nucleophile, the coordinated CO 2 molecule essentially retains its original linear
geometry or undergoes only slight distortion from linearity. This suggests that the
interaction of CO 2 with an electrophilic center through one of the 1p g lone pairs is
not accompanied by any significant back-donation of electron density into the 2p u
orbitals of the heterocumulene, which, therefore, are left empty.
End-on (η
1 -OCO) coordination mode has remained elusive for a long time.
Nevertheless, this coordination mode is now well documented [12] and plays an
important role in biological systems, such as photosynthetic CO 2 fixation by
ribulose-1,5-biphosphate carboxylase-oxygenase (RuBisCO) [13a]. A crystallographic study on a deacetoxycephalosporin C synthase (DAOCS) mutant showed
the presence of electron density close to the iron center of the active site, which was
found to be consistent with the presence of a monodentate O-coordinated CO 2
molecule [13b].
7.1.3.2 Carbon Dioxide as C-Electrophile
η
1 -C coordination of CO 2 to metal centers has been clearly demonstrated [12]. The
structural data of the adducts indicate an important pouring of electron density from
metal-filled d-orbitals into the empty antibonding 2p u (6a1) orbital, which weakens
the C–O bonds and, as expected from the Walsh diagram (Fig. 7.4), causes a
remarkable bending of the coordinated CO 2 molecule. Analogous structural changes always mark the geometry of CO 2 molecule whenever the electrophilic carbon
atom of CO 2 is involved in an interaction with electron-rich species (hydride,
amines, alcohols, carbanions, etc.) [12].
This interaction also affects the nucleophilicity of the O-atoms of CO 2 . For
instance, in CO 2 adducts with amines or alcohols, the resulting carbamate or carbonate anions, under suitable conditions, can easily react with C-electrophiles to
give organic carbamates [14] or carbonates, [15] respectively (see Chaps. 9, 10).
7.1.3.3 Amphoteric Reactivity of Carbon Dioxide
Most frequently, however, the reactivity of CO 2 fully reveals the intrinsically
amphoteric nature of the molecule. Side-on (η
2 -C, O) coordination of CO 2 to metal
106
7 Properties of the Carbon Dioxide Molecule
approximation (CEPA) method with a correlation consistent basis set, are,
respectively, equal to 174.3° and 117.6°, while the O–C, C–OH, and O–H bond
lengths are, respectively, of 112.4, 122.9, and 98.4 pm. Theoretical investigations
agree on the fact that the OCO backbone of this molecule is slightly bent, with a
trans configuration and the C–O and C–OH bond distances being, respectively,
shorter and longer than what is observed in the non-protonated molecule [6b, c, 11].
The positive charge is mainly localized on the carbon and hydrogen atoms with the
following repartition: C, +0.584; H, +0.350; O, +0.012; and (H)O, +0.054. The
alternative C-protonation affords a cyclic dioxyril cation with a very weak O–O
(1.733 Å) bond and a very high positive charge (+0.89) at the central CH unit.
Alternative to the protonation is the interaction with metal cations: several
examples of O-coordinated CO 2 to metal centers are known. The available structural data show unambiguously that, whenever CO 2 behaves exclusively as an
O-nucleophile, the coordinated CO 2 molecule essentially retains its original linear
geometry or undergoes only slight distortion from linearity. This suggests that the
interaction of CO 2 with an electrophilic center through one of the 1p g lone pairs is
not accompanied by any significant back-donation of electron density into the 2p u
orbitals of the heterocumulene, which, therefore, are left empty.
End-on (η
1 -OCO) coordination mode has remained elusive for a long time.
Nevertheless, this coordination mode is now well documented [12] and plays an
important role in biological systems, such as photosynthetic CO 2 fixation by
ribulose-1,5-biphosphate carboxylase-oxygenase (RuBisCO) [13a]. A crystallographic study on a deacetoxycephalosporin C synthase (DAOCS) mutant showed
the presence of electron density close to the iron center of the active site, which was
found to be consistent with the presence of a monodentate O-coordinated CO 2
molecule [13b].
7.1.3.2 Carbon Dioxide as C-Electrophile
η
1 -C coordination of CO 2 to metal centers has been clearly demonstrated [12]. The
structural data of the adducts indicate an important pouring of electron density from
metal-filled d-orbitals into the empty antibonding 2p u (6a1) orbital, which weakens
the C–O bonds and, as expected from the Walsh diagram (Fig. 7.4), causes a
remarkable bending of the coordinated CO 2 molecule. Analogous structural changes always mark the geometry of CO 2 molecule whenever the electrophilic carbon
atom of CO 2 is involved in an interaction with electron-rich species (hydride,
amines, alcohols, carbanions, etc.) [12].
This interaction also affects the nucleophilicity of the O-atoms of CO 2 . For
instance, in CO 2 adducts with amines or alcohols, the resulting carbamate or carbonate anions, under suitable conditions, can easily react with C-electrophiles to
give organic carbamates [14] or carbonates, [15] respectively (see Chaps. 9, 10).
7.1.3.3 Amphoteric Reactivity of Carbon Dioxide
Most frequently, however, the reactivity of CO 2 fully reveals the intrinsically
amphoteric nature of the molecule. Side-on (η
2 -C, O) coordination of CO 2 to metal
106
7 Properties of the Carbon Dioxide Molecule