R3M+ cations

The structure of isolated R3M+ cations in the gas phase was the subject of several computational studies. Significant difficulties are associated with calculations for molecules containing the heavier elements, particularly Sn and Pb. NMR chemical shift calculations require consideration of relativistic effects (spin-orbit coupling). A discussion of these difficulties and of effective core potentials developed for these calculations is beyond the scope of this review. 

Several major advances have been made in the field of R3M+ cations (M = Ge, Sn, Pb) in recent years, including the preparation of free cyclotrigermenium cations and possibly close to free trimesitylgermyl and trimesitylstannyl cations. However, many challenges remain, for example, a structural characterization of free R3M+ cations (M = Ge, Sn, Pb). In many respects these cations appear to be even more challenging than the silyl cations, due to their weaker and longer bonds to carbon. Presently inadequate tools which need to be improved are computational methods for the NMR chemical shifts of the Sn and Pb nuclei. 

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