The mechanisms of hydrolysis of a model cationic dinitrosyl iron complex with a prototypic thioformaldehyde ligand have been studied using the density functional theory and polarizable continuum water model. The free-energy calculations have predicted that the associative mechanism of the thioformaldehyde ligand removal has a ∼34 kJ mol-1 lower activation barrier in water than the dissociative mechanism. The additional estimates of chemical hardness have provided useful qualitative characterization of the thio ligands binding.