Substitution at Pentacoordinate Phosphorus

Even though the nucleophilic subtitution reactions in the phosphoranes series have been studied experimentally fairly well [181, 185], practically no data on their stereochemistry were reported. This question was addressed in the semiempirical CNDO/2 and MINDO/3 calculations [186] using as model examples a series of simple phosphoranes attacked by a hydride ion  

The calculated MERP of nucleophilic addition may be explained when examining the structure of the lowest-vacant MO of phosphorane PH5. With D31, symmetry of the molecule, this orbital is fully localized in the equatorial plane of LXVI, which agrees with the results of ab initio calculations on PH 5 [187]. Clearly, the optimal overlapping of the occupied orbital of the nucleophile H and the vacant p-orbital of phosphorus in the lowest vacant MO of PH5 will be achieved precisely on the approach route shown in Fig. 5.14. Increased overlapping is aided by the D3h C2v distortion of the PH5 molecule near the reaction zone for LXVII  

The calculations [186] have shown that in the case of substituted phospho-ranes favored would be an approach of the nucleophile from the inside of the angle R(i,PR(2) made by the most electropositive groups R,i, and R(2 of the 

The general conclusions on the mechanism of associative nucleophilic substitution at pentacoordinate phosphorus come down to two points 

1) The first step of the substitution reaction consists in adding the nucleophile to form a stable hexacoordinate phosphorus ion owing to the equatorial approach of the nucleophile to the central atom, a cis-position of the most electronegative groups is realized in the intermediate octahedral ion. 

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There is a growing realization that hexacoordinate phosphorus species may play an important role in organophosphorus chemistry (M3). The intriguing problem of the extent to which the hexacoordinate compounds are involved as intermediates in the substitution at pentacoordinate phosphorus atom still remains unresolved. 

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