Phosphorus pentafluoride, bonding

Phosphorus pentafluoride (bp -84.5 °C) is most easily prepared by reaction of PCI5 with ASF3 or HF, but many other methods have been used. The molecule has a trigonal bipyramidal stmcture in the gas phase with longer axial bonds (158 pm compared to 153 pm for the equatorial fluorines). Pseudorotation in molecules was first recognized in PF5 to... 

Draw the Lewis structure for phosphorus pentafluoride (PF5), in which aU five F atoms are bonded directly to the P atom. 

Phosphorus pentafluoride (Dyh). A Lewis structure for the PF molecule requires ten electrons in the valence shell of the phosphorus atom and the use of is, ip, and id orbitals and five tr bonds. It is impossible to form five bonds in three dimensions such that they are all equidistant from one another, but the trigonal bipyramidal (Fig. 6. le) and square pyramidal arrangements tend to minimize repulsions. Almost every five-coordinate molecule (coordination compounds excepted) which has been carefully investigated has been found to have a trigonal bipyramidal structure. The structure of the PF3 molecule is shown in Fig. 6.le (sp d hybrid). The bonds are of two types axial, the linear F—P—F system and equatorial, the three P—F bonds forming a trigonal plane. 

There is an interesting report of carbonyl substitution in the reaction between phosphorus pentafluoride and MeSiH2Co(CO)4 (114, 7). There is evidence that PF5 is reduced by the silicon-hydrogen bonds yielding HCo(CO)4, fluorosilane, and trifluorophosphine the latter subsequently displaces carbon monoxide from the hydridocarbonyl complex. 

All of the bonds we have encountered thus far have been formed by the valence electrons, the highest-energy s- and p-orbital electrons in the atom. Atoms of elements in the third period and higher can have more than four electron pairs surrounding them. This is accomplished by tf-orbitals forming covalent bonds. Phosphorus pentafluoride, PF5, places five electron-pair bonds around the phosphorus atom six pairs surround sulfur in SEgi... 

Phosphorus pentafluoride has five bonding pairs of electrons and no lone pairs. The repulsion between the electron pairs results in the most stable structure being a trigonal bipyramid (Figure 4.20). Three of the fluorine atoms lie in the same plane as the phosphorus atom. The bond angles FPF within this plane are 120". Two of the fluorine atoms lie above and below this plane at 90" to it. 

When the third period is considered, three out of seven atoms show exceptions to the rule of eight aluminum, phosphorus, and sulfur. Aluminum chloride, phosphorus pentafluoride, and sulfur hexafluoride are typical covalent compounds. They must, therefore, contain six, ten, and twelve bonding electrons, respectively. 

Phosphorus forms a tri- and a pentafluoride (Fig. 16). The coordination geometry of the trifluoride is pyramidal and that of the pentafluoride trigonal bipyramidal as expected. The average bond distance in the pentafluoride is about 1 pm shorter, and the mean bond energy is 10% lower than in the trifluoride. [The reader might compare with the bond energies of divalent and tetravalent Group 14 element chlorides listed in Table 14. The bonds in the tetrachlorides are 8% to 18% weaker... 

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