Electrospray ionisation mass spectroscopic characterisation of isopolyoxo-niobates and -tantalates

The electrospray ionisation mass spectrometric (ESI MS) characterisation of potassium isopolyoxo-niobate and -tantalate solutions (at pH 11, 10−3 M), including the mixed Nb/Ta oxometalate system, primarily yields ions that are based on the M6O19 moiety (with some protonation of most species being evident). In both positive ion and negative ion modes, moieties of the form [AmM6O19]n±, [AmM6O18]n±, [AmM6O17]n± and [AmM6O16]n− (with A=H and/or K, M=Nb and/or Ta) are evident, with the last three types of species being derived from a formal [M6O19]8− species by the progressive loss of three O2− ions. In negative ion mode, polymeric species containing 4, 8, 10, 12 and 14 metal atoms are also observed in the single metal systems, many of which can be related by {M2O5} polymerisation units. Most of these are hydrated species, such as [A6M8O24(H2O)n]2− and [A3M12O33(H2O)n]3− (both with M=Nb and Ta), [A5Ta12O34(H2O)n]3− and [A3Nb14O38(H2O)n]3−, and are not observed in the ‘mixed’ Nb/Ta oxometalate system.