All reagents and solvents were supplied by Sigma Aldrich Chemical Company Ltd, Thermo Fisher Scientific, and Cambridge Isotope Laboratories. All materials were used without further purification. Preparation: Precursors, the Li, Na and K salts of [PW11O39] 7-were prepared as described in previous literature, the synthesis of the compounds analysed in this manuscript, the Li, Na and K salts of [Mn(H2O)PW11O39] 5-, [Fe(H2O)PW11O39] 5-, [Co(H2O)PW11O39] 5-have been adapted from published methods, modifications in the synthetic methods are detailed in the SI.