Competition between Intra and Intermolecular Pnicogen Bonds: Complexes between Naphthalene Derivatives and Neutral or Anionic Bases

The PnF₂ (Pn=P,As,Sb,Bi) on a naphthalene scaffold can engage in an internal pnicogen Pn⋅⋅⋅N bond (PnB) with an NH₂ group placed close to it on the adjoining ring. An approaching neutral NH₃ molecule can engage in a second PnB with the central Pn, which tends to weaken the intramolecular bond. The presence of the latter in turn weakens the intermolecular PnB with respect to that formed in its absence. Replacement of the external NH₃ by a CN⁻ anion causes a fundamental change in the bonding pattern, producing a fourth covalent bond with Pn, which rearranges into a trigonal bipyramidal motif. This addition disrupts the internal Pn⋅⋅⋅N pnicogen bond, recasting the PnF₂⋅⋅⋅NH₂ interaction into an NH⋅⋅⋅F H-bond.