Hyperconjugation vs. Apicophilicity in Trigonal Bipyramidal Phosphorus Species
Cramer, C. J.; Gustafson, S. M.
J. Am. Chem. Soc. 1993, 115, 9315.
The global minimum calculated for trihydroxyphosphorane (CCSD(T)/cc-pVTZ//MP2/cc-pVDZ level) has all three hydroxyl groups equatorial in the trigonal bipyramid. For trihydroxyphosphoranyl, the equivalent structure, which places the highly apicophobic unpaired electron axial, is a local minimum only slightly above the global minimum in which a single hydroxyl is axial and the unpaired electron is equatorial. Thus, hyperconjugative interaction between equatorial ligands can overcome the tendency for more electronegative groups to prefer axial positions in trigonal bipyramids even in phosphoranyl radicals.
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