Optimization of the design of half-sandwich organometallic RuII arene complexes as anticancer agents depends on control of ligand exchange reactions. We have studied the aqueous chemistry of complexes containing O,O-chelate rings. The presence of the four-membered O,O-chelate ring from acetate (AcO) in [η6-p-cymene)Ru(AcO)Cl] was confirmed by X-ray crystallography, but in solution the acetate ligand was labile and the hydroxo-bridged dimer [((η6-p-cymene)Ru)2(μ-OH)3]+ readily formed. The dimer was relatively unreactive towards 9-ethyl guanine. The tropolonato (trop) complex [(η6-p-cymene)Ru(trop)Cl] was stable in aqueous media and the X-ray crystal structure of the aqua adduct [(η6-p-cymene)Ru(trop)(H2O)]CF3SO3, containing a five-membered O,O-chelate ring from trop, was determined. [(η6-p-cymene)Ru(trop)Cl] reacted with guanosine to form N7 adducts and with adenosine to form both N7 and N1 adducts. Competitive reactions with guanosine and adenosine gave rise to guanosine:adenosine adducts in a ca. 1.3:1 mol ratio.