Why Are [P(C6H5)4]+N3- and [As(C6H5)4]+N3- Ionic Salts and Sb(C6H5)4N3 and Bi(C6H5)4N3 Covalent Solids? A Theoretical Study Provides an Unexpected Answer

A recent crystallographic study has shown that, in the solid state, P(C6H5)(4)N-3 and As(C6H5)(4)N-3 have ionic [M(C6H5)(4)]N-+(3)--type structures, whereas Sb(C6H5)(4)N-3 exists as a pentacoordinated covalent solid. Using the results from density functional theory, lattice energy (VBT) calculations, sublimation energy estimates, and Born-Fajans-Haber cycles, it is shown that the maximum coordination numbers of the central atom M, the lattice energies of the ionic solids, and the sublimation energies of the covalent solids have no or little influence on the nature of the solids. Unexpectedly, the main factor determining whether the covalent or ionic structures are energetically favored is the first ionization potential of [M(C6H5)(4)]. The calculations show that at ambient temperature the ionic structure is favored for P(C6H5)(4)N-3 and the covalent structures are favored for Sb(C6H5)(4)N-3 and Bi(C6H5)(4)N-3, while As(C6H5)(4)N-3 presents a borderline case.