The quantum-chemical DFT calculations of fragments of molybdophosphoric Keggin polyoxoanion and its iron-substituted derivative are performed. It is shown that fragment calculations reproduce well local geometry and charge distribution in the whole polyoxoanion. It is found that the equilibrium Fe-II + Mo-VI <-> Fe-III + Mo-V can be realized in iron-substituted Keggin polyoxoanions. It gives an interesting example of participation of heteroatoms in polyoxoanion addenda in the electron delocalization processes.