Stability of metal ion complexes formed with methyl phosphate and hydrogen phosphate

Abstract

The acidity constants of methyl phosphoric acid, CH3OPO(OH)2, and orthophosphoric acid, HOPO(OH)2, and the stability constants of the 1 : 1 complexes formed between Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ and methyl phosphate, CH3OPO32–, or hydrogen phosphate, HOPO32–, were determined by potentiometric pH titration in aqueous solution (25 °C;I = 0.1 M, NaNO3). On the basis of previously established log K versus pKa straight-line plots for the complexes of simple phosphate monoesters and phosphonate derivatives, R-PO32–, where R is a noncoordinating residue, it is shown that the stability of the M(CH3OPO3) complexes is solely determined (as one might expect) by the basicity of the –PO32– residue. It is emphasized that the mentioned reference lines may also be used to reveal increased complex stabilities, 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 observation is attributed to a more effective solvation, including hydrogen bonding, than is possible with CH3OPO32– species.