On Covalent N₂H₆: A High-Energy, Non-Lewis, Local Minimum That May (Not) Exist

Saturated molecules of period 2 p-block atoms, such as CH₄, NH₃, and H₂O with full octets, are not expected to dimerize by covalent bond formation. A computational examination of NH₃ as an exception is carried out. Quite high in energy relative to the isolated monomers, the covalently bound local singlet hexahydridodinitrogen (N₂H₆) minimum of D_3d symmetry on the potential energy surface of the ammonia pair is probed. The structure and bonding of this double-octet dimer system are confirmed, but we find that the location of the species on the potential energy surface is highly sensitive to model chemistry, especially for post-Hartree-Fock methods. Of several singlet dimer pairs examined herein, no other system with period two p-block central atoms formed stable covalent minima. The prospect for dissociation through competing channels provides strong reasons for pessimism about any realization of the N₂H₆ species. Yet, the observed energy lowering for N₂H₆, which is strengthened by Rydberg bonding, challenges chemists to continue pressing against the perceived limits of chemical bonding.