TERNARY COMPLEXES IN SOLUTION WITH HYDROGEN PHOSPHATE AND METHYL PHOSPHATE AS LIGANDS

The stability constants of the 1:1 complexes formed between Cu(Arm)(2), where Arm = 2,2'-bipyridyl or 1,10-phenanthroline, and methyl phosp hate, CH3OPO32-, or hydrogen phosphate, HOPO32-, were determined by po tentiometric pH titration in aqueous solution (25 degrees C; I = 0.1 M , NaNO3). On the basis of previously established log K versus pK(a) st raight-line plots (D. Chen et al., J. Chem. Soc., Dalton Trans., (1993 ) 1537-1546) for the complexes of simple phosphate monoesters and phos phonate derivatives, R-PO32-, where R is a non-coordinating residue, i t is shown that the stabilities of the Cu(Arm)(CH3OPO3) complexes are solely determined by the basicity of the -PO32- residue. In contrast, the Cu(Arm)(HOPO3) complexes are slightly more stable (on average by 0 .15 log unit) than expected on the basicity of HPO42-; this is possibl y due to a more effective solvation including hydrogen bonding, an int eraction not possible with coordinated CH3OPO32- species. Regarding bi ological systems the observation that HOPO32- is somewhat favored over R-PO32- species in metal ion interactions is meaningful.