The tetranuclear ruthenium arene compound [(cym)4Ru4(2)Cl6]Cl2 (3) (cym = η6-p-cymene, 2 = 1,2-bis(di-N-methylimidazol-2-ylphosphino)ethane) was prepared and characterised by one- and two-dimensional NMR techniques. Its cytotoxicity against four different cell lines was determined and, with an approximate IC50 of >100 μM 3 can be regarded as non-toxic. Its partition coefficient in n-octanol/water (log D7.4) was also determined. The structures of complex 3 as well as of the related compound [(cym)2Ru2(4)Cl2]Cl2 (5) (4 = 1,2-bis(di-N-methylimidazol-2-ylphosphino)ethane dioxide) were determined by single crystal structure analysis. Upon oxidation in protic solvents, ligand 2 shows P–C bond cleavage reactions to yield P,P′-bis(N-methylimidazol-2-yl)ethylene diphosphinic acid (6).