MULTICOMPONENT POLYANIONS .46. CHARACTERIZATION OF THE ISOMERIC KEGGIN DECAMOLYBDODIVANADOPHOSPHATE IONS IN AQUEOUS-SOLUTION BY P-31 AND V-51 NMR

The Keggin decamolybdodivanadophosphate ions have been studied in aque ous 0.6 M Na(Cl) medium by potentiometry and P-31 (202.5 MHz) and V-51 (131.55 MHz) NMR spectrometry. An assignment of all detected resonanc es of decamolybdodivanadophosphate species to individual Keggin isomer s is proposed. The rive positional isomers of alpha-[Mo10V2PO40]5- wer e identified and characterized. Two minor [Mo10V2PO40]5- species, prob ably beta-Keggin isomers, were also observed. The pK(a) values of the conjugate monoprotonated species, [HMo10V2PO40]4-, have been precisely determined from the P-31 and V-51 chemical shift vs pH behavior, usin g alpha-[Mo11VPO40]4- as internal standard. These pK(a) values are in the range 0-0.5 at 25-degrees-C and in the range 0.8-1.1 at 90-degrees -C. From quantitative NMR integral data the relative abundances of the alpha isomers at equilibrium were evaluated. The distribution among t hem is close to statistical, with little measurable change between 25 and 90-degrees-C.