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

A central ZF 3 molecule (Z = P, As, Sb, Bi) is allowed to interact with a number of nucleophiles exemplified by NCH, NH 3 , 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 < NH 3 < NC - order for the three bases. The maximum number of ZBs depends on both the nature of the base and pnicogen atom. PF 3 and AsF 3 can pnicogen bond with only a single CN - ; SbF 3 and BiF 3 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 NH 3 . The latter NH 3 maximum can be extended to four ZBs but only for BiF 3 . The fourth ZB is somewhat longer and weaker than the others, and the entire (H 3 N) 4 ···BiF 3 complex relies partially on secondary interactions for its stability.