space group P2,/n. The structure consists of trigonal bipyramidal molecules in which the apical Sb-Br bond is very long [2.965(1) A]; the apical and mean equatorial Sb-C distances are 2.1 51 (9) and 2.1 02(9) A, respectively. The perchlorate [tetragonal, a = 12.670(2), c = 6.71 l ( 2 ) A, space group /a] crystallises with isolated ions of almost perfect tetrahedral local symmetry; in the cation, the Sb-C distance is 2.095(2) A. The structures of a range of tetraphenylantimony(v) derivatives Ph,SbX are discussed in terms of the basicity/nucleophilicity of X. Crystals of Ph,SbBPh, are tetragonal, a = 16.272(3), c = 13.703(3) A, with an / lattice: structure solution was attempted, without success, in all possible space groups. The SbPh; and BPh, ions are scrambled.Following the structure determinations 1 , 2 of two stibonium ylides (1) and (2), in which the lengths of the unique ylidic Sb-C bonds were found to be 2.042(3) 8, and 2.049(4) A, respectively, significantly shorter than the corresponding Sb-aryl bonds, it was necessary to establish, for comparative purposes, the Sb-C distance in an unperturbed tetrahedral Ph,Sb+ cation as a benchmark structural parameter.Nearly all compounds of stoicheiometry Ph,SbX, the structures of which have been reported, have been found to contain five-co-ordinate antimony; these include derivatives with X = OH,, OMe,5 OC(0)H,6 Cl,7 OSO,Ph,* and ON=C(CH,)N0.9 The carbonate (Ph,Sb),CO, (3) contains both five-and six-co-ordinate antimony. l oThe use of a poorly nucleophilic counter-ion of high symmetry offers the best chance of observing an unperturbed Ph,Sb' unit of regular tetrahedral configuration: the [Cu2-C1,]'ion fulfils the first condition, but not the second and the cations in (Ph,Sb),[Cu,CI,] are ' accordingly somewhat