The reaction of the ruthenium arene dimers [( η 6 -arene)Ru( μ -Cl)Cl] 2 (where arene=benzene or p -cymene) with the ligands 4-benzylidene-3,5-di(2′-pyridyl)-4-amino-1,2,4-triazole ( L 1 ), 2-methoxybenzylidene-3,5-di(2′-pyridyl)-4-amino-1,2,4-triazole ( L 2 ), 4-methylbenzylidene-3,5-di(2′-pyridyl)-4-amino-1,2,4-triazole ( L 3 ) and indole-3-carbaldehyde-3,5-di(2′-pyridyl)-4-amino-1,2,4-triazole ( L 4 ) in a 1:2 ratio gives the new complexes [( η 6 -arene)RuCl(L)] + [arene=C 6 H 6 (with L=L 1 ( 1 ), L 2 ( 3 ), L 4 ( 7 ), with PF 6 − as a counter ion, and L 4 ( 6 ), with Cl − as a counter ion) or p -cymene with L=L 1 ( 2 ), L 2 ( 4 ), L 3 ( 5 ), L 4 ( 8 ) with PF 6 − as a counter ion]. All complexes were fully characterized using 1 H and 13 C NMR, elemental analyses, UV/Vis and IR spectroscopy. The single crystal X-ray structures of ligand L 2 and complex 1 have been determined. The structure of 1 has the Ru atom coordinated with the arene group and to the N , N ′-bidentate ligand and to the Cl atom. The arene group occupies the apex, while the ligand and the Cl atom are at the base of a pseudo-octahedral three-legged piano stool. The cytotoxicity of these mononuclear complexes was established in the human epithelial colorectal adenocarcinoma cell line (Caco-2) and for selectivity in the non-cancerous human embryonic kidney cell line (HEK293), using 5-fluorouracil (5-FU) as the reference anticancer drug. Compounds 1 and 7 were relatively inactive toward the Caco-2 tumor cells (IC 50 >200), while complexes 2–5 showed moderate anti-proliferative properties (IC 50 >100–200). Compound 6 , however, displayed better anti-proliferative properties with an IC 50 value lower than that of the reference drug, 5-FU, and was therefore further investigated for its antimicrobial activity against six Gram-positive and four Gram-negative bacteria.