Apical positions electronegative substituents

The most common structures of arsenic compounds are tetrahedral and pyramidal, which are similar when the sterically active lone pair is counted. Tetrahedral symmetry holds the potential for chirality and indeed many chiral organoarsenic compounds have been prepared. Arsenic may also use d orbitals for (d-d)n bonding and for hybridization with s2 and p3 orbitals, resulting in trigonal bipyramidal or octahedral structures. In the former the more electronegative substituents occupy the apical position. 

Pentacovalent phosphorus compounds normally have trigonal-bipyraimidal structures. In such structures the more electronegative substituents show a strong preference for the apical positions. With these facts in mind, suggest an explanation for the following data obtained by nuclear magnetic resonance studies. 

C.N.-5. Phosphoranes. These are uncommon derivatives, following the usual rules of phos-phoranes that stability requires the presence of multiple electronegative substituents. Unlike the lower coordination numbers, this condition develops two sets of bond angles around phosphorus, 120° in the equatorial plane and 90° in the two apical positions. 

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