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)₃Cl₂L] (L = DMSO (3), L-H₃CSO(CH₂)₂CH(NH₂)CO₂H) (6a); D,L-H₃CSO(CH₂)₂CH(NH₂)CO₂H (6b); 3-NC₅H₄(CH₂)₂SO₃Na (7); 4-NC₅H₄(CH₂)₂SO₃Na (8); PTA (9); DAPTA (10); H₃CS(CH₂)₂CH(OH)CO₂H (11); CNCMe₂CO₂Me (12); CNCMeEtCO₂Me (13); CN(c-C₃H₄)CO₂Et) (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 CO₂ 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^II(CO)(H₂O)₄ at the His15 binding site. Soakings with 7 (4UWU) produced the metallacarboxylate [Ru(COOH)(CO)(H₂O)₃]⁺ 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⁻ 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.