Ligand site exchange

The fluxionality of organosilicon compounds has been the subject of two studies. At low temperature, the H and NMR spectra of 

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The hexacoordinate silicon complexes discussed here are highly flexible compounds, like some other hypercoordinate silicon compounds,1 6 and undergo a variety of fluxional reorganization reactions, observable by temperature-dependent NMR spectroscopy. The dimethylamino-coordinated bis-chelates 30-38 are particularly suitable for an NMR study of ligand-site exchange processes (inter- or intramolecular), because of the pairs of diastereotopic V-methyl groups, which under certain... 

Fig. 13. Schematic representation of ligand-site exchange in 32 (1,2-shift, topomerization) through a bicapped-tetrahedral intermediate or transition state. Upper part (Ph,Cl) interchange lower part (0,0) interchange.45 Reproduced with permission from the American Chemical Society.

Si-Cl Bond Distances and Free-Energy Barriers for the Low-Energy Ligand-Site Exchange in Hexacoordinate Complexes... 

Solvent Effect on Activation Free Energies for the Ligand-site Exchange... 

The dynamic behavior of tris-chelates was of considerable interest due to the additional constraint of a third bidentate ligand, relative to the extensively studied bis-chelates, with two monodentate ligands (Section III.A.3). Also the question of whether N-Si dissociation takes place during multi-step ligand-site exchange processes was considered. 

Few variable-temperature studies of Yb(II) systems have appeared, but it is apparent that dynamic processes such as solvent exchange, speciation equilibria and ligand site exchange occur frequently and complicate the interpretation of solution NMR data. The solvent choice is important in Yb NMR sp>ectroscopy. If the solvent is a poor Lewis base it may lower the number of coordinating bases (e.g. free and coordinated base present), thereby... 

In conclusion, two consecutive ligand-site exchange processes were measured in hexacoordinate complexes 1-4, and were assigned to (X,C1) and (0,0)-1,2-shifts (in order of increasing activation barrier). Possibly also Si-N cleavage follows in these compounds at higher temperature. N H coupling was observed for the first time across the N—>Si coordinative bond. 

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