Pentacoordination and ionic character: crystal structure of Ph4SbBr

Abstract

Ph4SbBr (P21/n, a = 12.282(2) Å, b = 10.656(1) Å, c = 16.156(1) Å, β = 104.92(7)°, Z = 4) crystallizes with the same space group as Ph4SbCl, but the two compounds are not isostructural. The structure of Ph4SbBr consists of layers of Ph4SbBr molecules with the Br atoms almost exactly in the [Formula: see text] planes. The coordination figure of the Sb atom is a trigonal bipyramid (TBP) with Br in an axial position. The Sb—Br distance, 2.950(1) Å is the longest reported for Sb(V) to date. The Sb(V)—Hal bond lengths and the Ph4SbBr and Ph4SbCl structures are compared in detail. Analysis of a large sample of Ph4EX (E = P, As) structures and of all the available R4EX (E = Sb, Bi; R = Ph, Me) structures shows that the former are always ionic, R4E+X−, whereas in the latter the observed coordination figures represent R4EX trigonal bipyramids with various degrees of axial distortion. The inverse variation of the axial bond lengths (the trans effect) in the TBP Sb compounds can be quantified as Sb—Cax ~ 2.4(Sb—X)−1/9 to a very high degree of correlation. This and other geometric relationships show that there exists a limiting E—X bond length (~3.3 Å for E = Sb and ~3.5 Å for E = Bi when X is oxygen) at which the TBP coordination becomes unstable and changes over to a tetrahedral R4E. This change is accompanied by a rearrangement of the structure to an ionic R4E+X− solid (of high symmetry where permitted by the size and shape of the anion) with [Formula: see text] separations no smaller than ~4 Å when E = Sb or Bi and X = O. The distribution of the CEC angles in the Ph4EX (E = P, As) sample is analyzed; the mean (uncorrected) P—C and As—C bond lengths in Ph4E+ are found to be 1.789(11) and 1.902(12) Å, respectively. Keywords: crystal structure, organoantimony compounds, Ph4SbBr, pentacoordination, tetraphenylantimony bromide.