Two bifunctional ligands, phenyl(carboxymethyl)phosphinate (ccp(2-)) and P,P'-diphenylmethylenediphosphinate (pcp(2-)), have been tested as chelating agents of beryllium(II). Both ligands have the same charge and a similar chelating structure, but whereas the 1:1 adduct of pcp(2-), Be(pcp)((HO)-O-2)(2), could be isolated as a white powder, no pure compound could be isolated from solutions containing beryllium(II) and ccp(2-). Instead, the solutions were examined by means of potentiometry and Be-9 NMR spectroscopy. Analysis of the potentiometric titration data with the program HYPERQUAD suggested the formation of the complex species BeL, [BeHL](+), [BeL2](2-), and [BeHL2](-) (L = ccp). The formation constants for these species were determined at 25 degreesC and / = 0.5 mol dm(-3) NaCIO4. The Be-9 NIVIR spectra are consistent with this model. The formation constants found for the ccp(2-) complexes are lower than those reported for related phosphonate ligands. However, the effective stability constant (which gives a better indication of the intrinsic coordinating capacity of the ligand at a particular pH) of the complex [Be(ccp)(2)](2-) at pH < 4 is greater than the effective constants of the corresponding phosphonoacetate and methylenediphosphonate complexes. The structure of Be(pcp)(H2O)(2) was determined by X-ray powder diffraction methods and consists of discrete molecules interconnected by an extended 2D network of hydrogen bonds, resulting in a stacking of double layers with a polar core and a lipophilic surface. Crystal data: C13H16BeO6P2, fw 339.21, monoclinic P2(1)/c, a 16.174(1) Angstrom, b = 8.979(1) Angstrom, c = 10.929(1) Angstrom, beta = 90.398(9)degrees, V = 1587.2(3) Angstrom(3), Z = 4.