Platinum ferrocenyl-phosphonate complexes, containing four-membered Pt-O-P(O)-O rings, have been synthesised by the reactions Of CiS-[PtCl2(PPh3)(2)] with the ferrocene-derived phosphonic acids Fc(CH2)(n)P(O)(OH)(2) (n = 0-2) [Fc = (eta (5)-C5H4)Fe(eta (5)-C5H5)] and 1,1 ' -Fe ' [P(O)(OH)(2)](2) [Fc ' = Fe(eta (5)-C5H4)(2)] in the presence of A920. The complexes have been characterised by NMR spectroscopy, together with crystal structure determinations on [Fc(CH2)nPO(3)Pt(PPh3)(2)] (n = 1, 2) and [1,1 ' -Fc ' {PO3Pt(PPh3)(2)}(2)] The complexes [Fc(CH2)(n)PO3Pt(PPh3)(2)] (n = 1, 2) show moderate activity against P388 leukaemia cells, whereas the parent phosphonic acids are inactive. (C) 2001 Elsevier Science B.V. All rights reserved.