Stereochemical Change in Five-Coordinate Systems

Most of the effort in the area of five-coordinate systems has been on trigonal bipyramid systems, such as PFj and Fe(CO)j. The interconversion is between the axial, a, and equatorial, e, positions. Studies on Fe(CO)j are described in section 4.5.1. 

The product appears to be rotated by 90° relative to the starting structure. Note that the intermediate structure is close to a square-based pyramid. 

Ugi et al. have suggested what is called the tumstyle mechanism which is described in reaction (4.9). This process is also consistent with the observations, because the two mechanisms cannot be distinguished experimentally. Theoretical arguments suggest that the pseudorotation mechanism provides the lower energy pathway for the interconvrasion.  

This mechanism is consistent with the NMR observations. If the H and the opposite P are considered as axial, then this process converts the axial P substituent to an equatorial position for each occurrence, and this is not the permutational equivalent of the pseudorotation process. 

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