Deciphering the Differences in Ambident Reactivity between the Cyanate, Thiocyanate Ions, and their P‐ and As‐Containing Analogues

Although the cyanate and thiocyanate anions belong to the most known textbook examples of ambident nucleophiles, the electronic factors that determine their markedly differing reactivities are still unclear. Their recently discovered P- and As-containing [PCX]– and [AsCX]–analogues (X: O, S, Se), whose ambident nature is practically unexplored, may serve as ideal basis for comparison to clarify these differences. Herein we present comprehensive theoretical investigations on the nucleophilic behaviours of the whole set of so far known [ECX]–(E: N, P, As, X: O, S, Se) anions, aiming for a systematic understanding of the reactivity patterns and deciphering the factors that govern the nucleophilic substitutions. Our results indicate that the SN2 reactions of the O-containing [ECO]– ions are thermodynamically preferred at the E pnictogen centres, while the kinetic contributions are only substantial for the N-containing [NCX]–anions. The ambident reactivities of the congeners that contain N or O significantly differ from those with P, As, S, or Se heteroatoms, in line with the inert s-orbital effect, characteristic for the heavier elements. By analysing the electronic structures and bonding patterns of the anions and relevant transition state structures, we offer clear explanations for the differing reactivities of the whole set of [ECX]–anions.