The 2:1 and 1:2 adducts of Au(I) and 1:2 adducts of Ag(I) with the diphosphine 2,3-bis(diphenylphosphino)maleic acid (dpmaa) have been prepared in high yields. Crystal structures have been determined for the neutral digold complex (AuCl)(2)(dpmaa)center dot 2thf (1) and the bis-chelated complex [Au(dpmaa)(2)]Cl center dot H2O center dot CH3OH (2). For 1, conformational rigidity imposed by the ethylenic bridge facilitates the formation of short intramolecular Au-Au contacts with no evidence of similar intermolecular contacts. Complex 2 crystallizes with [Au(dpmaa)(2)](+) cations hydrogen bonded through the carboxyl groups to a water molecule and chloride anion to form a H-bonded chain along the a axis. P-31 NMR titration of 1 with dpmaa in acetone shows conversion to 2 at Au:P-P ratios less than 1:1 indicating similar high thermodynamic and kinetic stabilities to other bis-chelated [Au(P-P)(2)](+) complexes containing 5-or 6-membered chelate rings. The ionic Au(I) complex 2 and the analogous Ag(I) complex [Ag(dpmma)(2)]NO3 (3) are highly water soluble. The in vitro cytotoxic activity of 2 was assessed against eight different cell lines and no significant activity was found. The solubility properties and solution behaviour of the complexes are compared to the analogous 1,2-bis(diphenylphosphino)ethane (dppe) complexes and the potential significance of these results to the antitumour properties of chelated 1:2 Au(I)diphosphine complexes are discussed. (c) 2005 Elsevier B.V. All rights reserved.