A common assumption that dimeric metal complexes in many catalytic systems represent a resting state and are not directly involved in catalytic processes was revised in a combined experimental and theoretical study. On-cycle participation of dimeric metal complexes, rather than typically assumed off-cycle involvement, was revealed, and advantageous performance in terms of improved selectivity was observed. The conceptual rationalization for the participation of dimeric species in the catalytic cycle was developed. The Pd-catalyzed hydrothiolation process (where strong Pd–S binding is well established and a persistent opinion for the inactive/poisoning role of dimeric species is presumed) was evaluated as a challenging system to test the concept. Activation of an (NHC)Pd(Cl)(acac) precatalyst (NHC ─N-heterocyclic carbene and acac ─acetylacetonate) under the reaction conditions produced monomeric (NHC)Pd(SPh)2 or dimeric (NHC)2Pd2(SPh)4 species depending on the steric bulkiness of the NHC ligand. Dimeric complexes…
