Heteroleptic hypervalent compounds

When discussing the molecular structures of heteroleptic hypervalent compounds we shall seek answers to the following questions  

The molecular structures of four heteroleptic fluoro-methyl-phosphoranes P(CH3)5 F , = 1,2,3, or 4, are shown in Fig. 15.9. Note that the F atom in the monofluoro derivative occupies an axial position, so do both the F atoms in the difluoro compound. When more F atoms are introduced, they enter equatorial positions. The only member of the series exibiting an axial methyl group is thus the monofluoro compound. Studies of the mixed 

Finally we turn our attention to the variation of bond distances. The structures shown in Fig. 15.9 show that 

Comparison of the molecular structures of As(CH3)s, As(CH3)3F2 and AsFs, and between Sb(CH3)s, Sb(CH3)3Cl2 and SbCls (Fig. 15.5) also show that replacement of methyl groups by more electronegative halogen atoms lead to shortening of both E-C and E-X bonds. (We have previously made the same observation regarding the bond distances the fluoro-methyl-silanes.) The structures of the phosphoranes also suggest that the axial bond distances are more sensitive to inductive effects than the equatorial. 

Before going on, we pass to note that crystallization of P(CH3)4F yields solid tetramethylphophonium fluoride, [P(CH3)4]E. 

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Fig. 15.5. Above the six known molecular structures of gaseous, homoleptic, hypervalent compounds of Group 15 elements. Below the structures of three heteroleptic compounds.

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