Influence of Charge and Coordination Number on Bond Dissociation Energies, Distances, and Vibrational Frequencies for the Phosphorus–Phosphorus Bond
2014
Chitnis, Saurabh S. | Whalen, J Marc | Burford, Neil
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.
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