Crystal structures of octaphenylcyclotetraphosphazene and octaphenylcyclotetra-arsazene

Abstract

The crystal structures of octaphenylcyclotetraphosphazene (Ph₈P₄N₄)(I) and of the arsenic analogue (Ph₈As₄N₄)(II) have been determined from diffractometer data by Patterson and Fourier methods and refined by least squares to R 0·046 [(I), 1000 observed reflexions] and 0·058 (II), 588 observed reflexions] respectively. The compounds are isostructural and have S₄ molecular symmetry with the ring conformation intermediate between the ideal boat (S₄) and the ideal saddle (D_2d) forms. The ring bonds for both compounds are equal and in each case the ring angle at nitrogen is small. The value for (II)(121·4°) is particularly low and may indicate a smaller degree of arsenic 4d orbital participation in the bonding than 3d orbital participation in (I). Crystals of both compounds are tetragonal, space group P2₁c, Z= 2 with unit cell dimensions: (I)a= 14·688(2), c= 9·755(2)Å; (II)a= 14·977(3), c= 9·708(3)Å.