ABINITIO MOLECULAR-ORBITAL STUDIES OF MO-95, O-17, AND S-33 CHEMICAL SHIELDING IN TRANSITION-METAL COMPOUNDS

Ab initio LORG (localized orbital-local origins) methods are used to calculate molybdenum, oxygen, and sulfur isotropic and tensor shielding data for the series of oxothiomolybdate anions [MoO(n)S(4-n)]2- (n = 0-4), Mo(CO)6, and MoNCl4-. With the exception of Mo(CO)6 the isotropic chemical shift data are overestimated, but the method provides an excellent correlation of the experimental chemical shift trends in this series of anions. With less than a 5% change in the diamagnetic shielding contributions, shielding in the oxothiomolybdates is dominated by the paramagnetic contributions. The sensivity of the paramagnetic Mo shielding contributions to the quality of the basis sets on both the metal and the ligands is no worse than that noted in distributed origins methods applied to first and second-row elements. Within the LORG algorithm Mo isotropic shieldings are examined in terms of the diamagnetic and paramagnetic contributions associated with inner shells, bonds, and lone pairs. Except for Mo(CO)6, where the paramagnetic contributions arise almost entirely from the t2g orbitals of the metal, the Mo shielding are dominated by paramagnetic contributions from the bonds and the lone pairs of the ligands. Calculated results for the ligand nuclei (S and O) follow the observed trends for O-17 and S-33 chemical shift data in the oxothiomolybdates. The excellent correlation, which is noted for all the calculated and experimental Mo-95, O-17, and S-33 chemical shift data with the computed Mo 4d atomic orbital populations, provides a useful method for relating the experimental data for the three different nuclei in the oxothiomolybdates. It seems likely that the same factors are implicated for the oxothio compounds of vanadium and tungsten as these exhibit the same chemical shift trends as the oxothiomolybdate anions.