Cyclophosphates as ligands for cobalt(III) in water.
Examination of cyclotriphosphate and cyclotetraphosphate as ligands for Co(III) in aqueous solutions revealed that cyclotetraphosphate affords stable complexes as a hemilabile ligand, while cyclotriphosphate exhibits facile hydrolysis.
Phosphato complexes of cobalt(III). I. General structural and hydrolytic properties
Phosphorus-32 tracer experiments have demonstrated that the phosphato ligand in [Co en2PO4]0 [Co(NH3)4PO4]O, [CO(NH3)4OH2HPO4]+, and [Co en2OH2.HPO4]+ undergoes slow substitution in acid and alkaline media at 40-60o and that a reversible hydrolysis equilibrium is established from pH 3 to 11. Outside this pH range, acid and base hydrolysis of all these phosphato complexes to diaquo or dihydroxo complexes is complete. The infrared spectra of [Co en2PO4]0 and [Co(NH3)4PO4]0 are consistent with a bidentate phosphato structure and with a monodentate phosphato ligand in CO(NH3)5PO4 and probably [Co(NH3)4OH2HPO4]+I-. The bidentate and monodentate complexes may be rapidly and reversibly interconverted but significant concentrations of the bidentate complexes only persist in the range pH 5.5-7.5. P.m.r, studies have established the predominantly cis configuration of the aquophosphato complexes. These complexes exhibit four acidity constants, one due to the aquo ligand, and three due to the monodentate phosphato ligand. The bidentate [Co en2PO4]0 exhibits a single acidity constant above pH 2. The aquophosphato complexes undergo both acid and base hydrolysis entirely by Co-O bond rupture. Acid hydrolysis of [Co en2PO4]0 also proceeds entirely by Co-O bond rupture but at pH 6.8 and in 0.25M NaOH hydrolysis proceeds by 60-65% Co-O bond rupture and by 30-35% P-O bond rupture.
Ion-pair formation of cyclic phosphate anions
Abstract An ion-exchange method, conductivity method and activity measurements have been applied to the investigation of ion-pair formation by cyclic phosphate anions in aqueuos solution. By conductivity measurements on their tetramethylammonium salts, the charges on the cyclic phosphate anions and ion equivalent conductivities at infinite dilution have been determined. From conductivities of sodium tetrameta-, hexameta- and octametaphosphate, association constants of ion pairs containing one sodium ion have been calculated by the method of Davies and Monk. From activity measurements on the sodium ions, an average number of sodium ions bound to each cyclic phosphate anion and association constants of the ion pairs containing one or more sodium ions have been calculated. It was concluded that the following ion pairs are present in dilute aqueous solutions of sodium metaphosphates; NaP4O123− in sodium tetrametaphosphate solution, NaP6O185−, Na2P6O184− and Na3P6O183− in sodium hexametaphosphate solution and NaP8O247−, Na2P8O246−, Na3P8O245− and Na4P8O244− in sodium octametaphosphate solution.