Phosphorus pentafluoride, structure

Polymerization of 4-bromo-6,8-dioxabicyclo[3.2.1 ]octane 2 7 in dichloromethane solution at —78 °C with phosphorus pentafluoride as initiator gave a 60% yield of polymer having an inherent viscosity of 0.10 dl/g1. Although it is not described explicitly, the monomer used seems to be a mixture of the stereoisomers, 7 7a and 17b, in which the bromine atom is oriented trans and cis, respectively, to the five-membered ring of the bicyclic structure. Recently, the present authors found that pure 17b was very reluctant to polymerize under similar conditions. This is understandable in terms of a smaller enthalpy change from 17b to its polymer compared with that for 17a. In the monomeric states, 17b is less strained than 17a on account of the equatorial orientation of the bromine atom in the former, whereas in the polymeric states, the polymer from 17b is energetically less stable than that from 17a, because the former takes a conformation in which the bromine atom occupies the axial positioa Its flipped conformation would be even more unstable, because the stabilization by the anomeric effect is lost, in addition to the axial orientation of the methylene group. 

Miscellaneous.—Hydridofluorophosphates have been prepared for the first time, and the anion (129) has been found to have an octahedral structure, with hydrogens trans.116 Phosphorus pentafluoride forms an adduct with biscyclopentadienyl titanium difluoride.116 The phosphorane (130) has been prepared as shown.117... 

Structural data are now available for the complete series of chlorofluorophosphoranes and are presented in Table 10. Phosphorus pentafluoride has been particularly well studied by electron diffraction, X-ray crystallography and microwave spectroscopy (Table 10), many studies being reinvestigations. As noted by Macho et al 255 and as can be seen from Table 10, the effect of successive substitution of fluorine in PF5 by chlorine... 

As indicated in Table 10-4, the four pentafluorides are all well known. Phosphorus pentafluoride, which may be prepared by fiuorinating PC15 with AsF3 or CaF2 is molecular, with a tbp structure in the solid state13 and under all other conditions. It is a colorless gas (bp -102°C), which reacts readily with Lewis bases such as amines and ethers, as well as F , to form six-coordinate complexes. The fluxional character of the molecule, whereby the axial and equatorial F atoms change places rapidly on the nmr time scale is generally explained as a Berry pseudorotation. 

Other aminophosphorane preparations that have been described include those of (86), (87), and (60). The trifluorophosphorane (86) is of interest because of the non-equivalence of the axial fluorines. Thus (86a) and (86b) show four lines for the F resonances of the axial fluorines. The novel phosphoranes (87) are prepared by treatment of the silylated tetramine with phosphorus pentafluoride or its simple derivatives (Scheme 5). The strain in the heterocyclic rings is presumably responsible for the adoption of a structure intermediate between sp and tbp. ... 

Physical and Theoretical Aspects.— Phosphorus pentafluoride continues to attract attention from those interested in theoretical aspects of structure, and from those more inclined to test the predictions of others. The most detailed ab initio calculations of the year lead to confirmation of Berry pseudorotation... 

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. 

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 structure in the gas phase with longer axial bonds (158pm compared to 153 pm for the equatorial fluorines). Pseudorotation in molecules was first recognized in PF5 to... 

The reaction between iVAr -bis(trimethylsilyl)sulphur di-imide and phosphorus pentafluoride gives a small amount of SaNgPFg, for which (77) is the suggested structure the n.m.r. spectrum shows two doublets,supporting... 

The gas phase molecular structures of phosphorus pentafluoride and pentachloride... 

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. 

Practice Problem A Draw the Lewis structure of phosphorus pentachloride (PCI5). Practice Problem B Draw the Lewis structure of antimony pentafluoride (SbF,). 

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