atoms which are characterized by the sp
3 hybridization and do not possess free
electron pairs. That rather excludes their potential basicity. Similarly, as it was
mentioned earlier here in the previous section, multivalent halogens may be characterized only by acidity properties but in that case the halogen centers possess lone
electron pairs.
The dual character mentioned few times here results in important consequences
that the same elements may be involved in the stabilization interaction; i.e. that
halogen-halogen, chalcogen-chalcogen, etc. interactions are possible. That is not all
since at least two different in nature connections may be mentioned here. For
example, the X halogen center may interact by its Lewis acid region (σ-hole) with
the Lewis base region (unshared electron pairs) of the X-center of another molecule
or both halogen centers being in the X…X contact may play the dual role, both may
act simultaneously as Lewis acid and Lewis base. Both cases of C–X…X–C
halogen-halogen interactions were described in early study of Fourmigue and Batali
[31] and they are presented in Scheme 15.1. In a case of the interaction designated
as I, the halogen atoms act simultaneously as Lewis acid and Lewis base centers.
Such centers may be sometimes related by symmetry, for example, in the crystal
structures such halogen atoms may belong to symmetry equivalent molecules. In a
case of the type II interaction one of halogens acts as the Lewis base and the second
one as the Lewis acid in spite of the fact that potentially both possess dual character.
Similar types of interactions are observed for other σ-hole bonds. For example,
P…P pnicogen interactions were analyzed in numerous complexes [54, 55]. For the
FH 2 P–PH 3 complex the σ-hole of the FH 2 P moiety situated in the elongation of the
F–P bond interacts with the lone electron pair of the PH 3 species; this corresponds
to the n(P) → σ PF
* orbital-orbital interaction within the NBO approach; in other
words the FH 2 P species reveals here the Lewis acid properties while the PH 3
molecule the Lewis base properties. One can expect the reverse situation, where the
FH 2 P and PH 3 molecules act as the Lewis base and the Lewis acid, respectively
with the corresponding n(P) → σ PH
* orbital-orbital interaction. However the P center
possesses the stronger Lewis acid properties in the FH 2 P molecule than in the PH 3
species since the n(P) → σ PF
* and n(P) → σ PH
* energies of interactions in the FH 2 P–
PH 3 complex are equal to 8.9 and 2.7 kcal/mol, respectively. Besides the FH 2 P σhole (elongation of the F–P bond) is characterized by the more positive EP than the
PH 3 σ-hole (elongation of H–P bond) [54, 55]. Figure 15.11 presents the molecular
graph of this complex with the reactive surface (∇ρ
2 (r) = 0 isosurface). One can see
that two molecules of the complex are oriented in such a way that lone electron pair
(the electron charge density concentration) is directed to the σ-hole region.
On the other hand the (FH 2 P) 2 dimer represents the situation of the equivalent
phosphorus atoms, both acting simultaneously as the Lewis acid and as the Lewis
base what corresponds to two equivalent n(P) → σ PF
* orbital-orbital interactions
(each characterized by the energy of 29.5 kcal/mol). Figure 15.12 presents the
molecular graph of this complex. The authors described also P…P interactions in
other complexes [55]; for example for the FH 2 P…P(OH)H 2 complex there are two
important orbital-orbital interactions: n(P) → σ PF
* and n(P) → σ P–OH
*
; however the
15 What Can Be Learnt from a Location of Bond Paths …
413
3 hybridization and do not possess free
electron pairs. That rather excludes their potential basicity. Similarly, as it was
mentioned earlier here in the previous section, multivalent halogens may be characterized only by acidity properties but in that case the halogen centers possess lone
electron pairs.
The dual character mentioned few times here results in important consequences
that the same elements may be involved in the stabilization interaction; i.e. that
halogen-halogen, chalcogen-chalcogen, etc. interactions are possible. That is not all
since at least two different in nature connections may be mentioned here. For
example, the X halogen center may interact by its Lewis acid region (σ-hole) with
the Lewis base region (unshared electron pairs) of the X-center of another molecule
or both halogen centers being in the X…X contact may play the dual role, both may
act simultaneously as Lewis acid and Lewis base. Both cases of C–X…X–C
halogen-halogen interactions were described in early study of Fourmigue and Batali
[31] and they are presented in Scheme 15.1. In a case of the interaction designated
as I, the halogen atoms act simultaneously as Lewis acid and Lewis base centers.
Such centers may be sometimes related by symmetry, for example, in the crystal
structures such halogen atoms may belong to symmetry equivalent molecules. In a
case of the type II interaction one of halogens acts as the Lewis base and the second
one as the Lewis acid in spite of the fact that potentially both possess dual character.
Similar types of interactions are observed for other σ-hole bonds. For example,
P…P pnicogen interactions were analyzed in numerous complexes [54, 55]. For the
FH 2 P–PH 3 complex the σ-hole of the FH 2 P moiety situated in the elongation of the
F–P bond interacts with the lone electron pair of the PH 3 species; this corresponds
to the n(P) → σ PF
* orbital-orbital interaction within the NBO approach; in other
words the FH 2 P species reveals here the Lewis acid properties while the PH 3
molecule the Lewis base properties. One can expect the reverse situation, where the
FH 2 P and PH 3 molecules act as the Lewis base and the Lewis acid, respectively
with the corresponding n(P) → σ PH
* orbital-orbital interaction. However the P center
possesses the stronger Lewis acid properties in the FH 2 P molecule than in the PH 3
species since the n(P) → σ PF
* and n(P) → σ PH
* energies of interactions in the FH 2 P–
PH 3 complex are equal to 8.9 and 2.7 kcal/mol, respectively. Besides the FH 2 P σhole (elongation of the F–P bond) is characterized by the more positive EP than the
PH 3 σ-hole (elongation of H–P bond) [54, 55]. Figure 15.11 presents the molecular
graph of this complex with the reactive surface (∇ρ
2 (r) = 0 isosurface). One can see
that two molecules of the complex are oriented in such a way that lone electron pair
(the electron charge density concentration) is directed to the σ-hole region.
On the other hand the (FH 2 P) 2 dimer represents the situation of the equivalent
phosphorus atoms, both acting simultaneously as the Lewis acid and as the Lewis
base what corresponds to two equivalent n(P) → σ PF
* orbital-orbital interactions
(each characterized by the energy of 29.5 kcal/mol). Figure 15.12 presents the
molecular graph of this complex. The authors described also P…P interactions in
other complexes [55]; for example for the FH 2 P…P(OH)H 2 complex there are two
important orbital-orbital interactions: n(P) → σ PF
* and n(P) → σ P–OH
*
; however the
15 What Can Be Learnt from a Location of Bond Paths …
413