Other tetrahedral molecules and ions

Further examples of tetrahedral molecules are shown at (a) and Others 

The phosphorus analogue of the ammonium ion, the phosphonium ion, has a regular tetrahedral structure in the salts (PH4)X. A n.d. study of PH4I gives P—H, T414 A and shows that the P—H bonds are directed towards I ions. The anion has 8 H neighbours at the corners of a distorted cube, 4 at 2-87 A and 4 at 3-35 A the atoms P—H—I are practically collinear (172°). Ions PX4 are included in the next section. 

In contrast to the pentachloride PBts crystallizes as (PBr4)Br, the P—Br bond length in the PBr 4ion being 2-15 A. The same cation occurs in PBr7/ formed by the action of Brj on PBrj, in which the anion is the linear (unsymmetrical) BrJ ion, and also presumably in compounds such as PBrel and PBrsICl. 

Of metallic salts M(PX6) only hexafluorophosphates appear to be known, and of these the salts with large cations such as NH4, K , Cs , Ba , and Co(NH3)6 , are the most stable in the solid state. The first three have been shown to crystallize with a NaCl-like packing of M and octahedral PF ions. The crystal structure of HPFg. 6 HjO is described on p. 544. In the PFj ion in this crystal P-F was found to be 1 73 A, but in NaPFg the length of this bond is apparently only T58 A. Moreover, in NaPFg. the ion was found to be distorted, with four 

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