STABILITY OF METAL-ION COMPLEXES FORMED WITH METHYL PHOSPHATE AND HYDROGEN PHOSPHATE

The acidity constants of methyl phosphoric acid, CH3OPO(OH)(2), and or thophosphoric acid, HOPO(OH)(2), and the stability constants of the 1: 1 complexes formed between Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, C u2+, Zn2+, or Cd2+ and methyl phosphate, CH3OPO32-, or hydrogen phosph ate, HOPO32-, were determined by potentiometric pH titration in aqueou s solution (25 degrees C; I = 0.1 M, NaNO3). On the basis of previousl y established log K versus pK(a) straight-line plots for the complexes of simple phosphate monoesters and phosphonate derivatives, RPO(3)(2- ), where R is a noncoordinating residue, it is shown that the stabilit y of the M(CH3OPO3) complexes is solely determined (as one might expec t) by the basicity of the -PO32- residue. It is emphasized that the me ntioned reference lines may also be used to reveal increased complex s tabilities, for example, for certain complexes formed with 8-quinolyl phosphate the occurrence of 7-membered chelates can be proven in this way; the same procedure is also applicable to complexes of nucleotides , etc. The M(HOPO3) complexes are slightly more stable (on average by 0.08 log unit) than it is expected from the basicity of HPO42-; this o bservation is attributed to a more effective solvation, including hydr ogen bonding, than is possible with CH3OPO32- species.