Influence of Charge and Coordination Number on Bond Dissociation Energies, Distances, and Vibrational Frequencies for the Phosphorus–Phosphorus Bond

We report a comprehensive and systematic
experimental and computational
assessment of the P–P bond in prototypical molecules that represent
a rare series of known compounds. The data presented complement the
existing solid-state structural data and previous computational studies
to provide a thorough thermodynamic and electronic understanding of
the P–P bond. Comparison of homolytic and heterolytic bond
dissociation for tricoordinate-tricoordinate, tricoordinate-tetracoordinate,
and tetracoordinate-tetracoordinate P–P bonds in frameworks 1–6 provides fundamental insights into
covalent bonding. For all types of P–P bond discussed, homolytic
dissociation is favored over heterolytic dissociation, although the
distinction is small for 21+ and 61+. The presence of a single cationic
charge in a molecule substantially strengthens the P–P bond
(relative to analogous neutral frameworks) such that it is comparable
with the C–C bond in alkanes. Nevertheless, P–P distances
are remarkably independent of molecular charge or coordination number,
and trends in values of d(PC) and νsymm(PC) imply that a molecular cationic charge is distributed over the
alkyl substituents. In the gas phase, the diphosphonium dication 32+ has similar energy to two [PMe3]+ radical cations, so that it is
the lattice enthalpy of 3[OTf]2 in the solid-state
that enables isolation, highlighting that values from gas-phase calculations
are poor guides for synthetic planning for ionic compounds. There
are no relationships or correlations between bond lengths, strengths,
and vibrational frequencies.