FORMATION AND PROTONATION OF AMINOPOLYPHOSPHONATE COMPLEXES OF ALKALINE-EARTH AND DIVALENT TRANSITION-METAL IONS IN AQUEOUS-SOLUTION

The formation and protonation of N-methyliminobis(methylenephosphonic acid) (midmp, H4L) and N,N-dimethylaminomethylphosphonic acid (dmamp, H2L) complexes with alkaline-earth (M = Mg2+, Ca2+, Sr2+ or Ba2+) and divalent transition metals (M = Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+) have been investigated at 25.0-degrees-C and at an ionic streng th of 0.1 mol dm-3 (KNO3) by means of potentiometry and P-31 NMR spect roscopy. The complex-formation constants, K(ML), for the alkaline-eart h metals decrease with increasing ion size. The order of K(ML) for the transition-metal complexes agrees with the Irving-Williams series for each ligand. The first protonation constants, K(MHL), of the alkaline earth-midmp and transition metal-dmamp complexes are quite large comp ared with the first protonation constants of the phosphonate group of the free ligand (the second protonation constants of the ligand K(H2L) ), and decrease with increasing complex-formation constant. The values of K(MHL) for the transition metal-midmp complexes are of the same or der of magnitude as the value of K(H2L) for the ligand. The second pro tonation constants K(MH2L) of all complexes are smaller than K(H2L) an d there is no significant difference among the complexes. The P-31 NMR spectra of the metal-ligand solutions have been measured at 25-degree s-C as a function of pH and the chemical shift of each species was eva luated. The results of the potentiometric and NMR studies suggest that the M-N bond of the ML complex is weakened by a decrease in the numbe r of methylenephosphonate groups of the ligand, i.e. protonation of mi dmp complexes of alkaline-earth and some transition metals and of all the dmamp complexes occurs at the nitrogen atom of the ligand, rupturi ng the M-N bond.