Berry twists mechanism

Pt and 119Sn NMR data show Pt(SnCl3)5 to be non-rigid (on the NMR timescale) down to 183 K, owing to an intramolecular process, possibly a Berry twist mechanism [132],... 

Fig. 7. Structures of five-coordinate Cu2+ from first principles molecular dynamics. A Berry twist mechanism for the interconversion of the two structures is shown (from left to right) the reorientation of the main axis of a square pyramidal configuration by pseudo-rotations via a trigonal bipyramidal configuration. The grey atoms in the plane of the trigonal bipyramid are all candidates for becoming apical atoms in a square pyramid.

Figure 74 (a) The Berry-twist mechanism connecting a trigonal bipyramidal and square pyramidal geometry (b) e modes of vibration (c) correlation of one-electron energy levels... 

From the energy point of view, the solvated complex [Cu(H20)5] " undergoes frequent transformations between square pyramidal and trigonal bipyramidal (TBP) configurations by a Berry twists mechanism (Scheme 3). 

The syn-syn, anti-anti exchange in square-planar complexes often depends on the ligand exchange process. This process may be explained by the angular twist mechanism in the pentacoordinate intermediate complex (Figure 7.4a) or by Berry s pseudorotation (Figure 7.4b). 

Figure 7.4. Syn-syn, anti-anti exchange (a) the angular twist mechanism of cis-trans isomerization and (b) Berry s pseudorotation mechanism of the exchange of equatorial and axial ligands in the trigonal bipyramid.

The question might be asked Are there similar mechanisms for changing the configuration of molecules without breaking bonds in molecules with coordination numbers other than 3 and 5 The answer is yes. One of the most important series of inorganic compounds consists of six-coordinate chelate compounds exemplified by the tris(ethylenediamine)cobalt(lIl) ion. Because of the presence of the three chelate rings, the ion is chiral and racemization can take place by a mechanism that is closely related to atomic inversion or Berry pseudorotaiion (the mechanism for six-coordination is termed the Bailar twist see Chapter 13k... 

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