A series of halfsandwich ruthenium sulfinato complexes [CpRu(PR'3)2(SO:R)] (R = Me, CH 2Ph, C2H4Ph, Ph, 4 -CöH4Me; PR'3 = PM e3, 1/2 dppm) with various electronic and steric environments around the ruthenium centre, have been prepared by insertion of SO2 into a ruthenium carbon bond, by a direct ligand exchange reaction, or by oxidation of thiolato complexes with 3-chloroperoxybenzoic acid. The chiral complexes [CpRu(C0 )(PPh3)(S0 2 R)] (R = Me, CH2Ph, Ph) were obtained similarly by oxidation of the corresponding thiolates with magnesium monoperoxyphthalate. Alkylation of the sulfinato complexes with oxonium salts [Rm 3 0 ]X (R" = Me, Et; X = B F 4 , PF6) gave ruthenium complexes of sulfinic acid esters, [C pRu(L)(L')(S(0)(0R ")R)]X in high yields and, for the chiral complexes, up to 82% de. The esters may be detached from the metal by ligand exchange with acetonitrile. Stronger nucleophiles such as I~ or SMe~ dealkylate the coordinated sulfinic acid esters.