A few ruthenium based metal carbonyl complexes, e.g. CORM-2 and CORM-3, have therapeutic activity attributed to their ability to deliver CO to biological targets. In this work, a series of related complexes with the formula Ru(CO) sub(3)Cl sub(2)L (L = DMSO (3), l-H sub(3)CSO(CH sub(2)) sub(2)CH(NH sub(2))CO sub(2)H) (6a); d,l-H sub(3)CSO(CH sub(2)) sub(2)CH(NH sub(2))CO sub(2)H (6b); 3-NC sub(5)H sub(4)(CH sub( 2)) sub(2)SO sub(3)Na (7); 4-NC sub(5)H sub(4)(CH sub( 2)) sub(2)SO sub(3)Na (8); PTA (9); DAPTA (10); H sub(3)CS(CH sub(2)) sub(2 )CH(OH)CO sub(2)H (11); CNCMe sub(2)CO sub(2)Me (12); CNCMeEtCO sub(2)Me (13); CN(c-C sub(3)H sub(4))CO sub(2)Et) (14)) were designed, synthesized and studied. The effects of L on their stability, CO release profile, cytotoxicity and anti-inflammatory properties are described. The stability in aqueous solution depends on the nature of L as shown using HPLC and LC-MS studies. The isocyanide derivatives are the least stable complexes, and the S-bound methionine oxide derivative is the more stable one. The complexes do not release CO gas to the headspace, but release CO sub(2) instead. X-ray diffraction of crystals of the model protein Hen Egg White Lysozyme soaked with 6b (4UWN) and 8 (4UWV) shows the addition of Ru super(II)(CO)(H sub(2)O) sub(4) at the His15 binding site. Soakings with 7 (4UWU) produced the metallacarboxylate Ru(COOH)(CO)(H sub(2)O) sub(3) super(+) bound to the His15 site. The aqueous chemistry of these complexes is governed by the water-gas shift reaction initiated with the nucleophilic attack of HO super(-) on coordinated CO. DFT calculations show this addition to be essentially barrierless. The complexes have low cytotoxicity and low hemolytic indices. Following i.v. administration of CORM-3, the in vivo bio-distribution of CO differs from that obtained with CO inhalation or with heme oxygenase stimulation. A mechanism for CO transport and delivery from these complexes is proposed.