How Many Pnicogen Bonds can be Formed to a Central Atom Simultaneously?

A central ZF3 molecule (Z = P, As, Sb, Bi) is allowed to interact with a number of nucleophiles exemplified by NCH, NH3, and NC– anion. The Z···N pnicogen bond (ZB) to a single base grows stronger for heavier Z atom: P < A < Sb < Bi and follows the NCH < NH3 < NC– order for the three bases. The maximum number of ZBs depends on both the nature of the base and pnicogen atom. PF3 and AsF3 can pnicogen bond with only a single CN–; SbF3 and BiF3 can interact with two anions but only weakly. The weak NCH nucleophile can engage in a maximum of two ZBs, while three ZBs occur for NH3. The latter NH3 maximum can be extended to four ZBs but only for BiF3. The fourth ZB is somewhat longer and weaker than the others, and the entire (H3N)4···BiF3 complex relies partially on secondary interactions for its stability.