. What is Pentahalide Phosphorous


Best Answer

The trigonal bipyramidal is not a regular structure. Electron diffraction on PF5 gas shows that some bond angles are 90 and others are 120, and the axial P-F bond lengths are 1.58 Å whilst the equatorial PF lengths are 1.53 Å. In contrast nmr studies suggest that all five F atoms are equivalent. This paradox may be explained quite simply. Electron diffraction gives an instantaneous picture of the molecule, whilenmr gives the picture averaged over several milliseconds. The axial and equatorial F atoms are thought to interchange their positions in less time than that needed to take the nmr. The interchange of axial and equatorial positions is called ‘pseudorotation’. PF5 remains covalent and keeps this structure in the solid state. However, PCl5 is close to the ionic-covalent borderline, and it is covalent in the gas and liquid states, but is ionic in the solid state. PCl5 solid exists as [PCl4]+ and [PCl6]- the ions have tetrahedral and octahedral structures respectively. In the solid, PBr5 exists as [PBr4]+ Br- and PI5 appears to be [PI4]+ and I- in solution.

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