We
have predicted, using a wide range of theoretical models, the
potential energy surfaces of dative bond stretching in some phosphine
haloboranes and closely associated analogues. It is shown that these
dative complexes demonstrate unusual bond stretching potentials that
are characterized by having multiple inflection points and are not
able to be fit to any traditional Morse or Lennard-Jones-type curve.
Specifically, in the case of Cl
3
B–PH
3
, this effect is so pronounced that the surface actually exhibits
two distinct minima. To the best of our knowledge, this is the first
example of such a unique bonding phenomenon reported for these species
and is explained by the competition between the energetic cost of
the required pyramidalization of the Lewis acid to form a dative bond
and the stabilization from the favorable attraction between the Lewis
acid and base. When the cost of pyramidalization of the Lewis acid
is high relative to the strength of the interaction between the acid
and base, the potential well associated with dative bonding is significantly
weakened and the result is a relatively flat potential energy surface
that is susceptible to the unusual characteristics described herein.