The acidity constants of the 2-fold protonated (1H-benzimidazol-2-yl-methyl)phosphonate, H-2(Bimp)(+/-), are given, and the stability constants of the M(H;Bimp)(+) and M(Bimp) complexes with the metal ions M2+ = Mg2+, Ca2+, Ba2+, Mn2+, Co2+, Cu2+, Zn2+, or Cd2+ have been determined by potentiometric pH titrations in aqueous solution at I = 0.1 M (NaNO3) and 25 degreesC. Application of previously determined straight-line plots of log K-M(Bi-R)(M) versus PKH(Bi-R)H for benzimidazole-type ligands, Bi-R, where R represents a residue which does not affect metal ion binding, proves that the primary binding site in the M(H;Bimp)(+) complexes is (mostly) N3 and that the proton is located at the phosphonate group; outersphere interactions seem to be important, and the degree of chelate formation is above 60% for all metal ion complexes studied, except for Zn(H;Bimp)(+). A similar evaluation based on log K-M(R-PO3)(M) versus pK(H(R-PO3))(H) straight-line plots for simple phosph(on)ate ligands, R-PO32-, where R represents a residue which cannot participate in the coordination process, reveals that the primary binding site in the M(Bimp) complexes is (mostly) the phosphonate group with all metal ions studied. In this case, the formation degree of the chelates varies more widely in dependence on the kind of metal ion involved, i.e., from 17 +/- 11% to nearly 100% for Ba(Bimp) and Cu(Bimp), respectively. For all the M(H;Bimp)(+) and M(Bimp) systems, the intramolecular equilibria between the isomeric complexes are evaluated in a quantitative manner. The fact that for Bimp(2-) the metal ion affinity of the two binding sites, N3 and PO32-, can be calculated independently, i.e., the corresponding micro 3 stability constants become known, allows us to present for the first time a method for the quantification of the chelate effect solely based on comparisons of stability constants which carry the same dimensions. This effect is often ill defined in textbooks because equilibrium constants of different dimensions are compared, which is avoided in the present case. For the M(Bimp) complexes, it is shown that the chelate effect is close to zero for Ba(Bimp) whereas for Cu(Bimp) it amounts to about four log units. This method is also applicable to other chelating systems. Finally, considering that benzimidazole as well as phosphonate derivatives are employed as therapeutic agents, the potential biological properties of Bimp, especially regarding nucleic acid polymerases, are briefly discussed.