Ru(II) eta6-arene complexes containing p-cymene (p-cym), tetrahydronaphthalene (thn), benzene (bz), or biphenyl (bip), as the arene, phenylazopyridine derivatives (C5H4NN:NC6H5R; R = H (azpy), OH (azpy-OH), NMe2 (azpy-NMe2)) or a phenylazopyrazole derivative (NHC3H2NN:NC6H5NMe2 (azpyz-NMe2)) as N,N-chelating ligands and chloride as a ligand have been synthesized (1-16). The complexes are all intensely colored due to metal-to-ligand charge-transfer Ru 4d6-pi* and intraligand pi -->pi* transitions (eta = 5000-63 700 M-1 cm-1) occurring in the visible region. In the crystal structures of [(eta6-p-cym)Ru(azpy)Cl]PF6 (1), [(eta6-p-cym)Ru(azpy-NMe2)Cl]PF6 (5), and [(eta6-bip)Ru(azpy)Cl]PF6 (4), the relatively long Ru-N(azo) and Ru-(arene-centroid) distances suggest that phenylazopyridine and arene ligands can act as competitive pi-acceptors toward Ru(II) 4d6 electrons. The pKa* values of the pyridine nitrogens of the ligands are low (azpy 2.47, azpy-OH 3.06 and azpy-NMe2 4.60), suggesting that they are weak sigma-donors. This, together with their pi-acceptor behavior, serves to increase the positive charge on ruthenium, and together with the pi-acidic eta6-arene, partially accounts for the slow decomposition of the complexes via hydrolysis and/or arene loss (t(1/2) = 9-21 h for azopyridine complexes, 310 K). The pKa* of the coordinated water in [(eta6-p-cym)Ru(azpyz-NMe2)OH2]2+ (13A) is 4.60, consistent with the increased acidity of the ruthenium center upon coordination to the azo ligand. None of the azpy complexes were cytotoxic toward A2780 human ovarian or A549 human lung cancer cells, but several of the azpy-NMe2, azpy-OH, and azpyz-NMe2 complexes were active (IC50 values 18-88 microM).