Ligand-site exchange processes

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... 

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. 

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. 

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... 

The proton transfer processes described above induce interesting effects on the geometry of these metal complexes upon protonation (see also Section II). If it is assumed that the equatorial cyano ligands form a reference plane and are stationary for any of these distorted octahedral cyano oxo complexes, the protonation/deprotonation process as illustrated in Scheme 3 is responsible for the oxygen exchange at the oxo sites. This process effectively induces a dynamic oscillation of the metal center along the O-M-O axis at a rate defined by kmv, illustrated in Fig. 15. This rate of inversion is determined by the rate at which the proton is transferred via the bulk water from the one... 

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