Ab initio MP2/aug′-cc-pVTZ
calculations have been carried
out on complexes H2C(X)P:PXH2, for X
= F, Cl, OH, CN, NC, CCH, H, CH3, and BH2. Three
sets of complexes have been found on the potential surfaces. Conformation
A complexes have A–P···P–A approaching
linearity, with A the atom of X directly bonded to P. Conformation
B complexes have A–P···P linear, but the P···PC
orientation of H2CPX may differ significantly from
linearity. Conformation C complexes are unique, since the pnicogen
bond involves π-electron donation and acceptance by H2CPX. The order of binding energies of the three conformations
of H2C(X)P:PXH2 is C > A > B,
with two
exceptions. Although the binding energies of conformation C complexes
tend to be greater than the corresponding conformation A complexes,
intermolecular distances in conformation C tend to be longer than
those in conformation A. Charge transfer stabilizes H2C(X)P:PXH2 complexes. The preferred direction of charge transfer is
from H2CPX to PXH2. In conformations
A and B, charge transfer occurs from a P lone pair on one molecule
to an antibonding σ* orbital on the other. However, in conformation
C, charge transfer occurs from the π orbital of H2CPX to the σ*P–A orbital of PXH2,
and from the lone pair on P of PXH2 through the π-hole
to the π*PC orbital of H2CPX. Changes
in charges on P upon complexation do not correlate with changes in 31P chemical shieldings. Computed EOM-CCSD spin–spin
coupling constants correlate with P–P distances. At each distance,
the ordering of 1pJ(P–P) is A > B > C. Binding
energies
and spin–spin coupling constants of conformation A complexes
of (PH2X)2, H2C(X)P:PXH2, and (H2CPX)2 with A–P···P–A
approaching linearity have been compared. For complexes with the more
electronegative substituents, binding energies are ordered (PH2X)2 > H2C(X)P:PXH2 > (H2CPX)2, while the order is
reversed
for complexes formed from the more electropositive substituents. A
plot of ΔE(PH2X)2/ΔE(H2CPX)2 versus ΔE[H2C(X)P:PXH2]/ΔE(H2CPX)2 indicates that there
is a systematic relationship among the stabilities of these complexes.
Complexes (PH2X)2 tend to have larger spin–spin
coupling constants and shorter P–P distances than H2C(X)P:PXH2, which in turn have larger coupling
constants and shorter P–P distances than (H2CPX)2, although there is some overlap. Complexes having similar
P–P distances have similar values of 1pJ(P–P).