The stability constants of the 1:1 complexes formed between methylphosphonylphosphate (MePP3-), CH3P(O)2--O-PO32-
, and Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ (M2+)
were determined by potentiometric pH titration in aqueous solution (25 C°
; l = 0.1 M,
NaNO3
). Monoprotonated M(H;MePP) complexes play only a minor role. Based on previously
established correlations for M2+
-diphosphate monoester complex-stabilities and diphosphate
monoester β-group. basicities, it is shown that the M(Mepp)-
complexes for Mg2+
and the ions of
the second half of the 3d series, including Zn2+
and Cd2+, are on average by about 0.15 log unit
more stable than is expected based on the basicity of the terminal phosphate group in MePP3-. In contrast, Ba(Mepp)-
and Sr(Mepp)-
are slightly less stable, whereas the stability for Ca(Mepp)-
is
as expected, based on the mentioned correlation. The indicated increased stabilities are explained
by an increased basicity of the phosphonyl group compared to that of a phosphoryl one. For the
complexes of the alkaline earth ions, especially for Ba2+, it is suggested that outersphere
complexation occurs to some extent. However, overall the M(Mepp)-
complexes behave rather as
expected for a diphosphate monoester ligand.
