Apicophilicity and BPR

Most compounds which pseudorotate at room temperatures have to be cooled to quite low temperatures before the process is stopped. Examples are (from NMR data), PF3Q2 at -80°C, PFClj at -138°C, Mc2NPF2 at -85°C, (Cp3)3P(OSiMe3)2 at -70°C and (CH2)4PF3 at -100°C. 

When two or more atoms or groups of low apicophilicity are attached to the P atom, pseudorotation may not occur at all as, for example, with Me2Pp3 d Me3Pp2 (Chapter 6.12). 

Apicophilicity, which appears to depend in a complex way on electronegativity, Ji-acceptor capacity and steric factors, has been the subject of much investigation. The following orders have been suggested  

The high apicophilicity of F is indicated by its preference for apical sites in many structures (4.263), (6.534) and (7.138). Equatorial positions are favoured by n-donor groups such as NRj or Ph probably because orbital overlap with d orbitals of P is easier in that position. 

Indications of equatorial tt-bonding in phosphoranes come from spectroscopic evidence of restricted rotation in such compounds as (13.117a) and (13.117b). For example, the F NMR spectra of PF4(SR) indicates shghtly different enviromnents for the two apical F atoms. This could occur only if there is restricted rotation about the P-S bond which allows the group R to he closer to one F atom than the other. 

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