The NBO analysis was performed to get an insight into the interplay
between pnicogen bonding and other interactions. The results of the NBO
analysis are summarized in Table S2. One can see that CT is much larger
in the pnicogen bond than that in the halogen bond, while it is smaller
in the pnicogen bond than that in the triel bond. It is generally
accepted that the larger CT corresponds to a stronger interaction[49], although for
different types of interaction there is no good correlation between CT
and binding energies. It is observed that the charge transfer is reduced
for both the halogen bond and pnicogen bond in
X…CN-Ph-CN…PO2F complexes, and a reverse
result is found in Y…Br-Ph-CN…PO2F
complexes. It should be mentioned that in the case of
BH3…CN-Ph-CN and
BH3…CN-Ph-CN…PO2F systems,
the complexes are considered as one molecule in the NBO program, and
thus the relevant charge transfer energies E (2)were not obtained. For BF3…CN-Ph-CN and
BF3…CN-Ph-CN…PO2F systems,
N lone pair donation occurs from the Lewis base into the empty p(π) orbital of B. The E (2) greatly decreases
from 152.28 to 14.93 kcal/mol from dimer to the trimer, which is
consistent with the increased B…N distance in the trimer in
comparison with the dimer.