In the present study, the potential anti-neoplastic properties of a series of ruthenium half-sandwich complexes of formula [Ru(η-arene)Cl(PRR(1-pyrenyl))] (η-arene = p-cymene and R = R = methyl for 1; η-arene = methylbenzoate and R = R = methyl for 2; η-arene = p-cymene and R = R = phenyl for 3; η-arene = methylbenzoate and R = R = phenyl for 4; η-arene = p-cymene, R = methyl and R = phenyl for 5; η-arene = methylbenzoate, R = methyl and R = phenyl for 6) have been investigated. The six structurally related organoruthenium(II) compounds have been prepared in good yields and fully characterized; the X-ray structures of three of them, i.e., 1, 2, and 4, were determined. Although the piano-stool compounds contain a large polycyclic aromatic moiety, viz. a 1-pyrenyl group, they do not appear to interact with DNA. However, all the piano-stool complexes show significant cytotoxic properties against five human cell lines, namely, lung adenocarcinoma (A549), melanoma (A375), colorectal adenocarcinoma (SW620), breast adenocarcinoma (MCF7), and nontumorigenic epithelial breast (MCF10A), with IC values in the micromolar range for most of them. In addition, the most active compound, i.e., 2, induces a remarkable decrease of cell viability, that is in the nanomolar range, against two human neuroblastoma cell lines, namely, SK-N-BE(2) and CHLA-90. Complexes 1-6 are all capable of inducing apoptosis, but with various degrees of magnitude. Whereas 1, 3, 5, and 6 have no effect on the cell cycle of A375 cells, 2 and 4 can arrest it at the G/M phase; furthermore, 2 (which is the most efficient compound of the series) also stops the cycle at the S phase, behaving as the well-known anticancer agent cisplatin. Finally, 2 is able to inhibit/reduce the cell migration of neuroblastoma SK-N-BE(2) cells.