The first heteropoly molybdate and first heteropolymetallate, yellow ammonium phosphomolybdate, (NH)PMoO was discovered by Berzelius in 1826. The phosphorus atom in the anion is termed the heteroatom, other heteroatoms are silicon and arsenic. The heteropoly-molybdenum blues have structures based on the Keggin structure. The blue colour arises because the near-colourless anion, such as the phosphomolybdate anion, , can accept more electrons (i.e. be reduced) to form an intensely coloured mixed-valence complex. This can occur in one electron or two electron steps. The reduction process is reversible and the structure of the anion is essentially unchanged.
: PMo + 4 e ⇌ PMoMo
The structure of the anion, PMoMo, has been determined in the solid state and is a β-isomer (i.e. with one of the four groups of edge-shared octahedra on the α-Keggin ion rotated through 60°). Similar structures have been found with silicon, germanium or arsenic heteroatoms.
The intense blue colour of the reduced anion is the basis for the use of heteropoly-molybdenum blues in quantitative and qualitative analytical techniques.
This property is exploited as follows:
*the sample to be analysed is reacted to produce the reduced blue heteropoly-molybdate in order to:
**detect the presence of a hetero atom in e.g. a spot test
** measure the amount of a hetero atom present in the sample colorimetrically
*the sample is added to a solution of the near colourless, unreduced complex in order to:
**detect the presence of a reducing compound e.g. a reducing sugar such as glucose
**measure the amount of a reducing compound in a two step procedure