Identifying the Isomers of Octahedral Complexes with 119Sn and 207Pb NMR Spectroscopy: A Computational Exercise

In this computational exercise, the Sn-119 and Pb-207 NMR spectra (data provided to the students) of the octahedral complexes [SnClnF6-n](2-) and [PbClnF6-n](2-), respectively, are examined. The pairwise-additivity model is applied to the NMR spectra of the complexes [MClnF6-n](2-) (n = 0, 1, 5, 6) to obtain the Cl-Cl, Cl-F, and F-F pairwise-additivity parameters for each set of complexes; these parameters then are employed to predict the appropriate chemical shift for each pair of isomers of the complexes [MClnF6-n](2-) (n = 2, 3, 4). By comparing the chemical shifts predicted by the pairwise-additivity model with the experimental values, the students are able to identify the one isomer of each pair that either is the only one that exists in solution or is the more abundant isomer. Molecular modeling (semiempirical PM3) is employed to see which geometric isomer of each pair is more stable. For both series of complexes, the only isomer in solution or the more abundant isomer proves to be the more stable one.