Five complexes of formula [Ru(eta(6)-C6H5CH2COOH)-(XY)Cl]Cl/Na (XY = ethylenediamine (1), o-phenylenediamine (2), phenanthroline (3), and oxalato (4)) and [Ru(eta(6):kappa(1)-C6H5CH2COO)-(tmen)]Cl (tmen = N,N,N',N'-tetramethylethylenediamine, 5C) have been synthesized and fully characterized. Five new X-ray crystal structures ([Ru(eta(6)-C6H5CH2COOH)(mu-Cl)Cl](2), 1, 3, 4, and 5C center dot PF6) have been determined, which are the first examples of ruthenium(II) structures with phenylacetic acid as arene ligand. Furthermore, 5C center dot PF6 is the first example of a five-membered tether ring with a Ru(eta(6):kappa(1)-arene:O) bond. The tether ring in these complexes opens in acidic pH (<5) and closes reversibly in aqueous solution. The chlorido open-form undergoes aquation, and the aqua adduct can be observed (prior to ring closure) by NMR The speciation has an attractive complexity in the pH range 0-12, showing interconversion of the three species (chlorido, aqua, and closed tether), dependent on the proton concentration and the nature of the XY chelating ligand. The closed tether version of 3, complex 3C, with sigma-donor/pi-acceptor phenanthroline as chelating ligand, opens up more readily (pH 4), while the tether ring in complex 5C hardly opens even at pH as low as 1. We have determined the pK(a) of the pendant carboxylic group and that of the aqua adduct (ca. 3 and ca. 7, respectively), which can be finely tuned by the appropriate choice of XY. Complexes 1 and 2, which predominate in their inactive (closed-tether) form in intracellular conditions, show some cytotoxic activity (IC50 130 and 117 mu M, respectively) in A2780 ovarian cancer cells. Complex 1 catalyzes the reduction through transfer hydrogenation of pyruvate to lactate and NAD(+) to NADH in the presence of formate as H-source. Co-incubation with sodium formate decreases the IC50 value of 1 in A2780 cancer cells significantly.