Abstract
The stability constants of ML-type complexes of the two linear triphosphate ligand anion analogues triphosphate ( {text{P}}_{ 3} {text{O}}_{10}^{5 - } ) and diimidotriphosphate ( {text{P}}_{ 3} {text{O}}_{ 8} ( {text{NH}})_{2}^{5 - } ) were investigated thermodynamically using potentiometric titrations according to Schwarzenbach’s procedure. The stability constants of the ML-type complexes of different divalent metal ions with {text{P}}_{ 3} {text{O}}_{ 8} ( {text{NH}})_{2}^{5 - } are larger than those of the corresponding complexes with {text{P}}_{ 3} {text{O}}_{10}^{5 - } because of the greater basicity of the imino group. The order of the stability constants for the ML-type complexes follows the Irving–Williams order, indicating that only non-bridging oxygen atoms are coordinated directly to the different metal ions in both ligands, and that the imino groups cannot participate in coordination to the metal ions. In the complexation reactions of the Ca2+, Sr2+, Ba2+– {text{P}}_{ 3} {text{O}}_{10}^{5 - } and Cu2+, Zn2+, Ni2+– {text{P}}_{ 3} {text{O}}_{ 8} ( {text{NH}})_{2}^{5 - } systems, each metal ion forms an enthalpically stable complex, and there was no suggestion of a conspicuous entropic effect based on the chelate effect. Monodentate complexes that are strongly coordinated with the ligands were therefore formed, whereas entropically stable bidentate complexes were formed in the complexation reactions of the Cu2+, Zn2+, Ni2+– {text{P}}_{ 3} {text{O}}_{10}^{5 - } and Ca2+, Ba2+, Sr2+– {text{P}}_{ 3} {text{O}}_{ 8} ( {text{NH}})_{2}^{5 - } systems. According to the HSAB concept, hard metal cations such as Ca2+, Ba2+ and Sr2+ should bind to the harder oxygen atoms rather than the softer nitrogen atoms of the imidopolyphosphate anions, preventing direct coordination to the imino nitrogen atom.Graphical Electronic supplementary materialThe online version of this article (doi:10.1007/s10953-013-0099-2) contains supplementary material, which is available to authorized users.
Highlights
Imidophosphates are phosphorus and nitrogen containing compounds that have been used as chemical fertilizers for some time
In our previous work [21,22,23,24], we showed that the affinities of a range of different cyclo-limidotriphosphate anions to protons and various divalent metal ions increased in a linear manner as the number of imino groups in the anions increased
Following from the determination of the log10 bML values using Schwarzenbach’s procedure, a good correlation has been found between the log10 bML and log10 K1 values
Summary
Imidophosphates are phosphorus and nitrogen containing compounds that have been used as chemical fertilizers for some time. It would be useful to study the complexation equilibria between the linear polyphosphate anions and a variety of different metal ions to find ways to improve the hydrolysis resistance of imidophosphate compounds. The enthalpy and the entropy changes that occurred during the ML-type complexation reactions of these anions (i.e., DH° and DS°) were determined from the temperature dependence of the complex stability constants, and the coordination structures of the complexes resulting from these anions were estimated. Complexation equilibria of the linear polyphosphate anions should provide useful information to enhance our understanding of the influence of different metal ions on the hydrolysis behaviors of imidopolyphosphates
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