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. The 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. This study presents 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 identifying the factors that govern the nucleophilic substitutions. The results indicate that the S(N)2 reactions of the O-containing [ECO](-) ions are thermodynamically preferred at the pnictogen centres E, 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, clear explanations are offered for the differing reactivities of the whole set of [ECX](-) anions. To serve for synthetic investigations, possible outcomes of nucleophilic substitutions are predicted, and the target molecules are expected to be versatile and useful synthons.