Electronic population on tungsten, molybdenum, and vanadium atoms and 183W, 95MO, and 51V NMR in polyoxometalates

For a large number (more than 60) of polyoxometalates (POM) of tungsten, molybdenum, and vanadium, the charges on metal cations of the coordination sphere have been calculated using the extended Huckel molecular orbital method, and obtained values are compared with the NMR chemical shifts of the corresponding nuclei. Depending on the local geometry around the metal cation formed by oxygen atoms that may be different in the large POM, the W-183 NMR chemical shifts (for tungstates in the range of +268 ppm to -300 ppm or even larger, if the shift of -670 ppm for phosphoperoxotungstate is taken into account) correlate with the charges created by the oxygen environment (q in a range from 3.893 to 3.521 and 3.280 for phosphoperoxotungstate). It was assumed that the charge created on the metal cation reflects the change in the energy of the virtual molecular orbitals participating in the magnetically dipole-allowed transitions affecting the paramagnetic contribution that is the determining term in the variation of the chemical shift. As a general rule, decreasing positive charge (increased electronic population) on the metal cation results in decreased chemical shift, i.e., corresponding to its shielding. Similar conclusions are made concerning the Mo-95 and V-51 NMR chemical shifts, if they are compared with the calculated charges for the corresponding nuclei. It was shown that some deviations from the observed tendency are often due to the fact that some bond lengths in the X-ray structure determination are out from their usual range. If allowance is made for the usual range of the bond lengths (mainly in the bond with the terminal oxygen atom) the observed trend "chemical shift/charge" is conserved. This situation should be distinguished from the case of the reduced diamagnetic POM, where an increase in the electronic population on the given atom results in its deshielding due to the increased paramagnetic electron circulation. Roughly, the W-183 NMR chemical shifts (in a range from +60 to +1500 ppm) linearly depend on the excess of the charge acquired by the given atom upon reduction. Calculations also show which atoms participate in LUMO of POM and consequently which cations are reduced. Such conclusions are consistent with observed chemical shifts of the reduced forms of POM.