The use of biomimetic complexes is a common strategy adopted to obtain valuable information on enzymatic structures and functions. Following our previous investigation on the dioxygenation process of a cysteinato ligand by an efficient [TpMe,Ph FeCysOEt] cysteine dioxygenase (CDO) biomimetic complex, a detailed DFT study is herein presented on two modified systems in which Fe(II) has been replaced by either Ru(II) or Os(II). The reaction mechanism by which such compounds catalyze the formation of cysteine sulfinic acid is fully elucidated and compared with the previous findings on Fe(II)-system. Contrary to what observed for iron, the reaction proceed via multistate reactivity patterns on competing singlet and triplet spin state surfaces. The outcomes of the present investigation allow to assess the changes in the whole process and the difference in reactivity on modification of the metal center while simultaneously revealing the crucial role played by iron in the original enzymatic mimetic.