Crystal Structure of Phosphorus Pentabromide

Abstract

IT has been shown¹ that phosphorus pentachloride has an ionic type of crystal structure which is tetragonal and contains [PCl₄]⁺ and [PCl₆]^π groups. Phosphorus pentabromide is entirely different in crystalline form. It is orthorhombic, and X-ray investigations give for the unit cell a = 5.6, b = 16.9, c = 8.3 A., with four molecules per unit cell. The space group is Pbcm and the four phosphorus atoms are therefore necessarily equivalent crystallographic-ally and chemically. The possibility of a structure containing two different complexes similar to those in phosphorus pentachloride does not arise in this case. It is impossible to fit in four molecules of PBr₅ of the trigonal bipyramidal form, and the appearance of the diffraction patterns is entirely unlike that of a molecular compound. Patterson and Fourier methods were used to elucidate the structure. It was found that the structure contains tetrahedral [PBr₄]⁺ groups located so that a plane of symmetry passes through each, and the fifth bromine is present as a bromide ion removed from the phosphorus at a distance about twice that of the four covalently linked atoms. It is natural to expect the stability of the octahedral complex to diminish with increasing radius of the halogen atoms so that the change-over from the constitution [PCl₄]⁺ [PCl₆]^π to [PBr₄]⁺ Br^π is not surprising.