The results of NMR and EPR studies are considered for large polyoxoanions of molybdenum and tungsten with paramagnetic and some diamagnetic centers which produce delocalized electrons. For various polyoxoanions the possible mechanisms of electron density transfer involving pand s-bonding, depending on hybridization, bond lengths, and geometric configuration are analyzed. The different contributions to the observed chemical shifts including contact, pseudocontact (dipolar ligand centered), dipolar (metal centered) and spin polarization terms are considered and sometimes evaluated. Formation of the delocalized electron pair or bipolaron is proven by 17 O-and 183 W-NMR. The changes observed in the 183 W-NMR spectra of polyanions with non-equivalent tungsten atoms with the delocalized electron pair are believed to be due to the existence of the spin bipolaron. A model of the circulation of the electron density in Co 3 + W 12 O 40 5due to the dynamic Jahn -Teller effect is developed. The possibility of spin-paired electron transfer from the 3d-cation into the ligand orbitals of the nucleus under investigation is proposed.