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

A recent crystallographic study has shown that, in the solid state, PC6H54N3 and AsC6H54N3 have ionic MC6H54N3- type structures, whereas SbC6H54N3 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 the ionic structures are energetically favored, is the first ionization potential of MC6H54. The calculations show that at ambient temperature the ionic structure is favored for PC6H54N3 and covalent structures are favored for SbC6H54N3 and BiC6H54N3, while AsC6H54N3 presents a borderline case.