A New Body - Stereochemical Change

As outlined at the commencement of Section 5.3, there are other types of reactions apart from substitution reactions. One of these of key importance involves what we can describe as a change in the stereochemistry. This may involve a change in gross shape of the molecule, such as transition from square planar to tetrahedral, or, more often observed, a change in the relative position or three-dimensional arrangement of ligands in a particularly-shaped complex. 

This system displays a relatively small activation barrier of 45kJmol-1, with a rate constant of 10s s-1 at 298 K. 

Molecules that exhibit stereochemical non-rigidity are said to display fluxional character. All molecules undergo vibrations about an equilibrium position that does not alter their average spatial location for a limited number, however, rearrangement that changes the configuration can occur. One of the commonest ligands, ammonia, is a simple example of the concept (Equation 5.49), since as a pyramidal molecule it can invert. 

This has a low energy barrier ( 25 kJ mol-1) and occurs rapidly ( 2 x 1010 s-1). However, for pyramidal PR3 compounds, the activation barrier has climbed to over 100 kJ mol-1, so inversion is very slow, sufficiently so to even allow enantiomer separation in the case of chiral phosphines. Coordination of the lone pair freezes the configuration. Examples of fluxional complexes tend to be found mostly, but not exclusively, amidst organometallic compounds. Spectroscopic methods, particularly NMR spectroscopy, assist in defining the fluxional behaviour. 

2 Change in the Mode of Ligand Coordination - Linkage Isomerization 

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