The dimers [{(ηⁿ-ring)MCl}₂(μ-Cl)₂] ((ηⁿ-ring)M = (η⁵-C₅Me₅)Ir, (η⁶-p-MeC₆H₄iPr)Ru) react with the modified cysteines S-benzyl-L-cysteine (HL1) or S-benzyl-α-methyl-L-cysteine (HL2) affording cationic complexes of the formula [(ηⁿ-ring)MCl(κ²N,S-HL)]Cl (1, 2) in good yield. Addition of NaHCO₃ to complexes 1 and 2 gave equilibrium mixtures of neutral [(ηⁿ-ring)MCl(κ²N,O-L)] (3, 4) and cationic [(ηⁿ-ring)M(κ³N,O,S-L)]Cl (6Cl, 7Cl) complexes. Similar mixtures were obtained in one-pot reaction by successive addition of the modified cysteine and NaHCO₃ to the above formulated dimers. Addition of the N-Boc substituted cysteine derivative S-benzyl-N-Boc-L-cysteine (HL3) and NaHCO₃ to the dimers [{(ηⁿ-ring)MCl}₂(μ-Cl)₂] affords the neutral compounds [(ηⁿ-ring)MCl(κ²O,S-L3)] ((ηⁿ-ring)M = (η⁵-C₅Me₅)Ir (5a), (η⁶-p-MeC₆H₄iPr)Ru (5b)). Complexes of the formula [(ηⁿ-ring)MCl(κ³N,O,S-L)][SbF₆] (6Sb–8Sb), in which the cysteine derivative acts as a tridentate chelate ligand, can be prepared by adding one equivalent of AgSbF₆ to the solutions of compounds 5 or to the mixtures of complexes 3/6Cl and 4/7Cl. The amide proton of compounds 8aSb and 8bSb can be removed by addition of NaHCO₃ affording the neutral complexes [(ηⁿ-ring)M(κ³N,O,S-L3_-H)] ((ηⁿ-ring)M = (η⁵-C₅Me₅)Ir (9a), (η⁶-p-MeC₆H₄iPr)Ru (9b)). Complexes 9a and 9b can also be prepared by reacting the dimers [{(ηⁿ-ring)MCl}₂(μ-Cl)₂] with HL3 and two equivalents of NaHCO₃. The absolute configuration of the complexes has been established by spectroscopic and diffractometric means including the crystal structure determination of (R_Ir,R_C,R_S)-[(η⁵-C₅Me₅)Ir(κ³N,O,S-L1)][SbF₆] (6aSb). The thermodynamic parameters associated with the epimerization at sulphur that the iridium compound [(η⁵-C₅Me₅)Ir(κ³N,O,S-L3_-H)] (9a) undergoes have been determined through variable temperature ¹H NMR studies.