The reactions of potassium salt of dithiocarbonate, R′OCS2K,4 (R′=Me, Et,nPr,nBu,iPr,iBu, -CH2Ph) with the low-spinctc-RuII(L)2Cl21,ctc-OsII(L)2Br22 andmer-[CoII(L)3](ClO4)2·H2O3 [L=2-(arylazo)pyridine, NC5H4-N=N-C6H4(R), R=H,o-Me/Cl,m-Me/Cl,p-Me/Cl;ctc: cis-trans-cis with respect to halides, pyridine and azo nitrogens respectively) in boiling dimethylformamide solvent resulted in low-spin diamagnetic RuII(L′)2,5, OsII(L′)26 and [CoIII(L′)2]ClO47 respectively (L′=o-S-C6H3(R)N=NC5H4N). In the complexes5, 6 and7 ortho carbon-hydrogen bond of the pendant phenyl ring of the ligands (L′) has been selectively and directly thiolated via the carbon—sulphur bond cleavage of4. The newly formed tridenate thiolated ligands (L′) are bound to the metal ion in a meridional fashion. In the case of cobalt complex (7), during the activation process the bivalent cobalt ion in the starting complex3 has been oxidised to the trivalent CoIII state. The reactions are highly sensitive to the nature and the location of the substituents present in the active phenyl ring. The presence of electron donating Me group at the ortho and para positions of the pendant phenyl ring with respect to the activation points can only facilitate the thiolation process. The complexes1c, 2c and3c) having chloride group at the ortho position of the active phenyl ring underwent the thiolation reaction selectively via the carbon—chloride bond activation process. The rate of carbon—chloride activation process has been found to be much faster compared to the C-H bond activation. The reactions are sensitive to the nature of the solvent used, taking place only in those having high boiling and polar solvents. The rate of the reactions is also dependent on the nature of the R′ group present in4, following the order: Me∼ Et>nPr>nBu>iPr>iBu≫-CH2Ph. The molecular geometry of the complexes in solution has been established by1H and13C NMR spectroscopy. The thiolated complexes (5, 6, 7) exhibit metal to ligand charge-transfer transitions in the visible region and intraligandπ-π* andn-π* transitions in the UV region. In acetonitrile solution the complexes display reversible MIII⇄MII reductions at 0.43 V for Ru (5a), 0.36 V for Os (6a) and −0.13 V for Co (7a) vs saturated calomel electrode (SCE).