The notorious reinforcement of the Cl···N halogen bond in the triads is reflected in
the change of a N-monosynaptic basin in the binary complex into a Cl–N disynaptic
in the triad. However, for the ternary complexes involving NH 3 and NCH the ELF
approach does not provide a clear picture of the weakening of the F–Cl chemical
bond since the population of the F–Cl disynaptic basins remains practically
unchanged or is a little larger in the triad. Only for the complex involving NHCH 2
this weakening is rather obvious since the formation of a new Be–F disynaptic
basin reduces the F–Cl ones to only two.
17.6 Conclusions
The CD approach provides a quantitative description of the charge transfer
occurring upon the formation of a beryllium bond between the Lewis base and the
BeX 2 moiety. For systems of suitable symmetry, the CD function can be easily
decomposed into additive contributions correlating with the different irreducible
representations of the molecular point group to which both the molecule and its
constituting fragments belong. This allows for the identification of the mechanisms
involved in the charge-transfer process and permits the quantification of possible
back-donations. Very importantly, this approach also provides a clear description of
cooperativity. This has been shown by the study of a series of ternary complexes
stabilized through beryllium and halogen bonds. By comparing the CD function of
the triads with that of the two binary complexes in which it can be decomposed, we
have shown that, in the former, the synergistic effect of halogen and beryllium bond
enhances the charge transfer associated with both these non-covalent interactions.
The comparison of the results obtained with the CD analysis with those provided by
QTAIM and ELF theories, shows a clear agreement as far as the analysis of
qualitative trends is concerned. This is well reflected for instance in the good
correlations between the amount of charge transferred calculated with the CD
approach and the electron density at the BCP of the linkages formed, calculated
accordingly to the QTAIM theory. However, the different viewpoints each of these
methodologies provide are clearly complementary, the CD being the only one that
permits to quantify the charge transfer from the Lewis base to the Lewis acid.
Acknowledgments This work has been also partially supported by the Ministerio de Economía y
Competitividad (Projects No. CTQ2012-35513-C02-01 and CTQ2013-43698-P), by the
STSM COST Action CM1204, by the Project FOTOCARBON-CM S2013/MIT- 2841 of the
Comunidad Autónoma de Madrid and by grants from MIUR (Rome, Italy) with the program
FIRB-Futuro in ricerca (RBFR1022UQ). Computational time at Centro de Computación Científica
(CCC) of the Universidad Autónoma de Madrid is also acknowledged.
17 Charge Transfer in Beryllium Bonds and Cooperativity …
483
the change of a N-monosynaptic basin in the binary complex into a Cl–N disynaptic
in the triad. However, for the ternary complexes involving NH 3 and NCH the ELF
approach does not provide a clear picture of the weakening of the F–Cl chemical
bond since the population of the F–Cl disynaptic basins remains practically
unchanged or is a little larger in the triad. Only for the complex involving NHCH 2
this weakening is rather obvious since the formation of a new Be–F disynaptic
basin reduces the F–Cl ones to only two.
17.6 Conclusions
The CD approach provides a quantitative description of the charge transfer
occurring upon the formation of a beryllium bond between the Lewis base and the
BeX 2 moiety. For systems of suitable symmetry, the CD function can be easily
decomposed into additive contributions correlating with the different irreducible
representations of the molecular point group to which both the molecule and its
constituting fragments belong. This allows for the identification of the mechanisms
involved in the charge-transfer process and permits the quantification of possible
back-donations. Very importantly, this approach also provides a clear description of
cooperativity. This has been shown by the study of a series of ternary complexes
stabilized through beryllium and halogen bonds. By comparing the CD function of
the triads with that of the two binary complexes in which it can be decomposed, we
have shown that, in the former, the synergistic effect of halogen and beryllium bond
enhances the charge transfer associated with both these non-covalent interactions.
The comparison of the results obtained with the CD analysis with those provided by
QTAIM and ELF theories, shows a clear agreement as far as the analysis of
qualitative trends is concerned. This is well reflected for instance in the good
correlations between the amount of charge transferred calculated with the CD
approach and the electron density at the BCP of the linkages formed, calculated
accordingly to the QTAIM theory. However, the different viewpoints each of these
methodologies provide are clearly complementary, the CD being the only one that
permits to quantify the charge transfer from the Lewis base to the Lewis acid.
Acknowledgments This work has been also partially supported by the Ministerio de Economía y
Competitividad (Projects No. CTQ2012-35513-C02-01 and CTQ2013-43698-P), by the
STSM COST Action CM1204, by the Project FOTOCARBON-CM S2013/MIT- 2841 of the
Comunidad Autónoma de Madrid and by grants from MIUR (Rome, Italy) with the program
FIRB-Futuro in ricerca (RBFR1022UQ). Computational time at Centro de Computación Científica
(CCC) of the Universidad Autónoma de Madrid is also acknowledged.
17 Charge Transfer in Beryllium Bonds and Cooperativity …
483