Stabilities of the Divalent Metal Ion Complexes of a Short-Chain Polyphosphate Anion and Its Imino Derivative

ABSTRACT The stability constants of ML-type complexes of the two linear triphosphate ligand anion analogues triphosphate ([Formula: see text]) and diimidotriphosphate ([Formula: see text]) 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 [Formula: see text] are larger than those of the corresponding complexes with [Formula: see text] 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+-[Formula: see text] and Cu2+, Zn2+, Ni2+-[Formula: see text] 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+-[Formula: see text] and Ca2+, Ba2+, Sr2+-[Formula: see text] 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 ABSTRACT