Super-pnicogen bonding in the radical anion of the fluorophosphine dimer

Highlights: • Pnicogen-bonded dimers can form an extremely strong bond if they are converted into a radical anion. • The strength of this super-pnicogen bond is quantitatively assessed from the local stretching force constants. • Super-pnicogen bonding is predicted on the basis of CASPT2 and CCSD(T) calculations. • Super-pnicogen bonding provides a possibility of converting pnicogen bonding into normal covalent bonding. The LUMO of the pnicogen-bonded fluoro-phosphine dimer has PP bonding character. Radical anion and dianion form relatively strong pnicogen bonds with some covalent character where however the dianion turns out to be a second order transition state. The binding energy of ${\left(\text{FPH}2\right)}_2^{-}$ is 30.4 kcal/mol (CCSD(T)/aug-cc-pVTZ; CASPT2(5,8): 30.7 kcal/mol) and the bond strength order measured with the local PP bond stretching force constant increases from 0.055 for the neutral dimer to 0.187 thus revealing that the stabilization of the radical anion is to a large extend a result of one-electron six-center delocalization. Pnicogen-bonded complexes have a stabilizing electron affinity.