The acidity constants of H-2(AcP) and H-2(AnP), where AcP2- = acetyl phosphate and AnP(2-) = acetonylphosphonate, as well as the stability constants of the 1:1 complexes formed between Mg2+, Ca2+, Sr2+ , Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ and AcP2- or AnP(2-) were determined by potentiometric pH titrations in aqueous solution (25 degrees C; I = 0.1 M, NaNO3). On the basis of previously established log K-M(R-PO3)(M) versus pK(H(R-PO3))(H) straight-line plots for phosph(on)ate ligands (R-PO32-), which only a simple -PO32-coordination (Sigel, H.; et al. Helv. Chim. Acta 1992, 75, 2634), it is concluded that the carbonyl oxygen atom of AcP2-and AnP(2-) participates in complex formation by giving rise to six-membered chelates for all of the;mentioned systems except for Ba(AnP) and Sr(AnP) the stability of which corresponds to a simple -PO32- coordination. The formation degree of the chelates formed by the alkaline earth ions and AcP2- is quite pronounced; it amounts, for example, for Mg(AcP) and Ca(AcP) to 41% +/- 5% and 48% +/- 8%, respectively. The corresponding results for Zn(AcP) and Cu(AcP) are 59% +/- 6% and 76% +/- 4%, respectively. The formation degree of the six-membered Cu(AnP) chelate increases in water containing 30% or 50% (v/v) 1,4-dioxane. This may also be surmised for the other divalent metal ions under conditions of lower solvent polarity arid poorer solvating properties than water. Such conditions exist in active-site cavities of enzymes, and the expected effects are shortly discussed. The indicated-measurements With AcP have only. become possible after the stabilities of the corresponding M(HPO4) complexes had been determined (Saha, A.; et al. J. Biol. Inorg. Chem; 1996, 1, 231) because AcP contains as an impurity always some phosphate; :this phosphate content had to be qualified and its effect was carefully considered: in the evaluations of the experimental data. Here is most probably the main reason why previously published results regarding the stabilities of the Mg(AcP) and Ca(AcP) complexes vary widely; stability constants of the other M(AcP) complexes have not been determined previously.