The new complexes [RuCl2(η6-p-cymene)(κP-Ph2PR)] [R = 4-C6H4OSiMe2tBu, 1; R = 4-C6H4Br, 2; R = OC([double bond, length as m-dash]O)CHCl2, 3; R = OPh, 4; R = O(2-C6H4SiMe2tBu), 5] and [Ru(C2O4)(η6-p-cymene){κP-Ph2PO(2-C6H4(SiMe2tBu))}], 6, were obtained in 83-98% yield from Ru(ii) arene precursors by three different synthetic strategies. The unprecedented phosphine Ph2P(O(2-C6H4SiMe2tBu)) was synthesized in 86% yield from 2-C6H4Br(OSiMe2tBu) and Ph2PCl, via intramolecular oxygen to carbon 1,3 migration of the silyl group (retro-Brook rearrangement). All the complexes were fully characterized by analytical and spectroscopic methods, and by single crystal X-ray diffraction in the cases of 3, 4, 5 and 6. Complexes 1-6 and the model compounds [RuCl2(η6-p-cymene)(κP-PPh3)] (Ru-PPh3) and [Ru(C2O4)(η6-p-cymene)(κP-PPh3)] (Ru-PPh3-O) underwent slow degradation in chloroform solutions upon air contact; the mixed valence complex [(η6-p-cymene)Ru(μ-Cl)3RuCl2(κP-PPh3)], 7, was isolated from a solution of Ru-PPh3 in CHCl3, and X-ray identified. The antiproliferative activity of 1-6 and Ru-PPh3, Ru-PPh3-O and [RuCl2(η6-p-cymene)(κP-PTA)] (RAPTA-C) was assessed towards the triple-negative breast cancer cell line MDA-MB-231, the ovarian carcinoma cell line A2780 and human skin fibroblasts (HSF). Complexes 1, 2, 5 and 6 displayed IC50 values significantly lower than that of cisplatin, with 2 providing a more potent cytotoxic effect on MDA-MB-231 and A2780 cancer cells compared to the noncancerous cell line (HSF). The stability of all complexes in DMSO/water solution was elucidated by NMR and conductivity measurements, and in particular 35Cl NMR spectroscopy was helpful to check the possible chloride dissociation. The stability studies suggest that the cytotoxic activity in vitro of the compounds is mainly ascribable to Ru(ii) species still bound to the phosphorus ligand.