lone electron pair region of P-center and it clearly ends at one of H-atoms of the
dihydrogen. The NBO results confirm that since the σ(H 2 ) → σ
* (P–F) and n
(P) → σ
* (H 2 ) orbital-orbital energies of interactions are equal to 0.2 kcal/mol and
0.6 kcal/mol, respectively showing the stronger Lewis acid properties of the
dihydrogen in that complex.
The interactions of molecular hydrogen with the PF(CH 3 ) 2 and PFH 2 species are
classified as weak ones thus there are only slight electron density shifts for the
corresponding complexes, -0.002 au and +0.008 au for the PFH 2 –H 2 and PF
(CH 3 ) 2 –H 2 complexes, respectively (NBO results). This means that for the PFH 2 –
H 2 complex there is the shift from the H 2 molecule while for the PF(CH 3 ) 2 –H 2
complex the shift to the dihydrogen is observed. Those shifts clearly confirm the
results presented earlier here of the Lewis base or Lewis acid properties of the
molecular hydrogen.
15.5 Summary
The analysis of the location of bond paths, of their trajectories and the distribution
of the electron density in species analyzed, especially if such a distribution is
discussed on the basis of the laplacian of the electron density, are very useful tools
to deepen the understanding of the nature of interactions. This is also important that
the analysis of the molecular graphs is often in line with the results of other
approaches. This is worth mentioning that the molecular graph may be treated as a
source of information on the location of atomic attractors (positions of atoms) and
on the location of bond paths corresponding to the preferable stabilizing
interactions.
Let us look at examples presented in this chapter; it is evident, that very often for
weak interactions the atom-atom contacts correspond to Lewis acid–Lewis base
centers’ contacts characterized by the opposite electrostatic potentials. The hydrogen bonded systems are examples of such a situation, or even more generally, the σhole bonds. This is interesting that the bond paths, for weak interactions, usually
connect the local maximum of EP with the minimum of EP. The complexes of the
BrF 5 characterized by the pentavalent bromine center are examples of that situation;
there are four equivalent local maxima of EP at the Br center situated symmetrically
around the fourfold axis of symmetry of the BrF 5 molecule. This is why the
complexes of BrF 5 with weak Lewis bases are not linear since the bond path crosses
one of the local maxima of bromine. However the interactions with stronger Lewis
bases like F
− or Cl
− anions lead to the meaningful changes in the electron charge
distribution what results in the formation of BrF 6
− and BrF 5 Cl
− complexes with the
F–Br…F
− and F–Br…Cl
− linear arrangements, respectively.
One can enumerate other examples where the molecular graphs inform of the
kinds and of the nature of interactions. The complexes of molecular hydrogen may
be mentioned here. The dihydrogen possesses Lewis acid and Lewis base characteristics since the positive electrostatic potential is observed at the H-atoms, at edges
15 What Can Be Learnt from a Location of Bond Paths …
429
dihydrogen. The NBO results confirm that since the σ(H 2 ) → σ
* (P–F) and n
(P) → σ
* (H 2 ) orbital-orbital energies of interactions are equal to 0.2 kcal/mol and
0.6 kcal/mol, respectively showing the stronger Lewis acid properties of the
dihydrogen in that complex.
The interactions of molecular hydrogen with the PF(CH 3 ) 2 and PFH 2 species are
classified as weak ones thus there are only slight electron density shifts for the
corresponding complexes, -0.002 au and +0.008 au for the PFH 2 –H 2 and PF
(CH 3 ) 2 –H 2 complexes, respectively (NBO results). This means that for the PFH 2 –
H 2 complex there is the shift from the H 2 molecule while for the PF(CH 3 ) 2 –H 2
complex the shift to the dihydrogen is observed. Those shifts clearly confirm the
results presented earlier here of the Lewis base or Lewis acid properties of the
molecular hydrogen.
15.5 Summary
The analysis of the location of bond paths, of their trajectories and the distribution
of the electron density in species analyzed, especially if such a distribution is
discussed on the basis of the laplacian of the electron density, are very useful tools
to deepen the understanding of the nature of interactions. This is also important that
the analysis of the molecular graphs is often in line with the results of other
approaches. This is worth mentioning that the molecular graph may be treated as a
source of information on the location of atomic attractors (positions of atoms) and
on the location of bond paths corresponding to the preferable stabilizing
interactions.
Let us look at examples presented in this chapter; it is evident, that very often for
weak interactions the atom-atom contacts correspond to Lewis acid–Lewis base
centers’ contacts characterized by the opposite electrostatic potentials. The hydrogen bonded systems are examples of such a situation, or even more generally, the σhole bonds. This is interesting that the bond paths, for weak interactions, usually
connect the local maximum of EP with the minimum of EP. The complexes of the
BrF 5 characterized by the pentavalent bromine center are examples of that situation;
there are four equivalent local maxima of EP at the Br center situated symmetrically
around the fourfold axis of symmetry of the BrF 5 molecule. This is why the
complexes of BrF 5 with weak Lewis bases are not linear since the bond path crosses
one of the local maxima of bromine. However the interactions with stronger Lewis
bases like F
− or Cl
− anions lead to the meaningful changes in the electron charge
distribution what results in the formation of BrF 6
− and BrF 5 Cl
− complexes with the
F–Br…F
− and F–Br…Cl
− linear arrangements, respectively.
One can enumerate other examples where the molecular graphs inform of the
kinds and of the nature of interactions. The complexes of molecular hydrogen may
be mentioned here. The dihydrogen possesses Lewis acid and Lewis base characteristics since the positive electrostatic potential is observed at the H-atoms, at edges
15 What Can Be Learnt from a Location of Bond Paths …
429