The light-protected reaction of [(η⁶-p-cymene)Ru^IICl₂]₂ with 1-(2-hydroxyethyl)piperazine in dry methanol, followed by addition of excess NH₄PF₆, afforded the complex [(η⁶-p-cymene)Ru^II(NH₃)₂Cl](PF₆) (1) in 47% yield. Attempts to use the same protocol for the synthesis of [(η⁶-p-cymene)Os^II(NH₃)₂Cl](PF₆) led to the isolation of the binuclear triply methoxido-bridged arene-osmium compound [{(η⁶-p-cymene)Os}₂(μ-OCH₃)₃](PF₆) (3). Both compounds were characterised by X-ray crystallography and ¹H NMR spectroscopy, and the ruthenium complex also by spectroscopic techniques (IR and UV-vis spectroscopies). The antiproliferative activity of complex 1in vitro was studied in A549 (non-small cell lung carcinoma), CH1 (ovarian carcinoma), and SW480 (colon carcinoma) cells and compared to that of [(η⁶-p-cymene)Ru^II(en)Cl](PF₆) (2). In contrast to the latter compound, 1 is only modestly cytotoxic in all three cell lines (IC₅₀: 293–542 μM), probably due to the instability of the diammine ruthenium complex in aqueous solution.