THEORETICAL-STUDY ON METAL NMR CHEMICAL-SHIFTS - ELECTRONIC MECHANISM OF THE SN CHEMICAL-SHIFT

Sn chemical shifts of the complexes SnMe4-xHx and SnMe4-xClx (x = 0-4) are studied theoretically by an ab initio molecular orbital method using larger basis sets than our previous ones. The calculated values of the Sn chemical shifts agree reasonably with the experimental values. The electronic mechanism of the Sn chemical shifts is analyzed. The Sn chemical shifts are mainly governed by the Sn valence p AO contribution to the paramagnetic term. The diamagnetic term is small and determined solely by the structural factor. The major factor affecting the p AO mechanism is the sigma-sigma* excitation energy and the minor factor the density distribution in the p AO's, and these two factors are canceling. The linear relationship in the SnMe4-xHx series is due to the large change in the excitation energy and the similarity between H and Me. The U-shaped relationship in the SnMe4-xClx series is due to the relatively small change in the excitation energy, the existence of the two lower states, and the anisotropic (for x = 1, 2, 3) and nonadditive inductive effects of Cl on the 5p AO density.