Experimental and Theoretical Approaches to the Protonation of Thiourea: A Convenient Nucleophile in Coordination Chemistry Revisited

The protonation of thiourea was reinvestigated in aqueous and non‐aqueous media using potentiometric, UV‐vis, NMR, MS spectroscopy, as well as DFT calculations. In aqueous medium, no evidence for protonation could be found in the pH range 0 to 7, in agreement with typical pKa values (acid dissociation constants of protonated thiourea) quoted in the literature of around ‐1.2. Most of the protonation constants quoted in the literature are incorrect and apparently refer to pKa values. Protonation of thiourea occurs at a much higher pKa value in methanol and in water‐dioxane mixtures, which must be related to the stabilization of the protonated species in such media.