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The existence of a heteropoly 18-molybdopyrophosphate was ascertained in both solid and solution states. The yellow complex was formed in solutions of 0.05 M (M=mol dm−3) Mo(VI)/5×10−3 M pyrophosphate/0.1–0.7 M HCl/60% (v/v) CH3CN at ambient temperatures, being isolated as tetraalkylammonium (R4N+) salts where R=n-C4H9 and C2H5. The presence of organic solvents was essential for the preparation of the complex. The 18-molybdopyrophosphate complex underwent a four-step reduction in 95% (v/v) CH3CN–water containing 0.1 M HClO4, yielding mixed-valence blue species by the electrochemical reduction at the respective steps of a reversible two-electron transfer. The heteropoly skeleton is retained in the blue complexes. The UV-visible and IR spectroscopic characteristics, and the formation conditions of the 18-molybdopyrophosphate complex were also elucidated.

The electrochemical properties of α- and β-dodecamolybdophosphates were characterized at the glassy carbon (GC) electrode. It was found that the β-isomer was only stable in neat organic solvents. When molybdophosphate was prepared in aqueous organic solutions, a mixture of α- and β-isomers formed.

The formation conditions of a yellow Dawson-type S2Mo18O624− anion have been investigated as functions of the concentrations of Mo(VI), H2SO4, organic solvents, the nature of the organic solvents used, temperature of the solution, and the reaction time. For a 50 mM Mo(VI)/50% (v/v) acetonitrile system, the yellow S2Mo18O624− anion formed in 0.2–1.0 M H2SO4, while a pale-yellow Mo6O192− anion did in 0.04–0.3 M H2SO4. The yellow heteropolyanion is electrochemically reduced in six successive steps to mixed-valence heteropoly blues at the glassy carbon electrode. Each of the first four waves corresponds to a reversible two-electron transfer. The mixed-valence blue species were characterized by their inherent visible and infrared (IR) spectra.