3 STEREOISOMERIC SQUARE-PYRAMIDAL COMPLEXES [MA3B2] - INVESTIGATIONS OF THE EXISTENCE AND THE STRUCTURES OF CRYSTALLINE DIMETHYL-SULFOXIDE COMPLEXES OF ANTIMONY AND BISMUTH TRICHLORIDES

The formation of solid complexes MCl3.n DMSO (M = Sb, Bi; n = 1 - 4) was reinvestigated. In each system, only two of four presumptive complexes could be isolated as crystalline solids: SbCl3 . DMSO (1a) was prepared for the first time, SbCl3 . 2 DMSO (1b), BiCl3 . 2 DMSO (2b) and BiCl . 3 DMSO (2c) were reproduced according to literature data. Evidence is presented as to the non-existence of BiCl3 . 4 DMSO, contrary to previous claims in the literature. A unit cell determination showed 2c to be structurally identical with the monomeric fac-octahedral complex BiCl3(DMSO)3 obtained fortuitously and described elsewhere [Z. anorg. allg. Chem. 620 (1994) 1037]. The compounds 1a (monoclinic, space group P2(1/c)), 1b (monoclinic, space group P2(1/n)) and 2b (monoclinic, space group C2) represent examples of each of the three possible geometric isomers of a square-pyramidal complex [Ma3b2]. In the formally 1/1 adduct 1a, which is in fact [Sb(1)Cl3(DMSO)2 . Sb(2)Cl3], the Sb(1) atom of the complex unit displays square-pyramidal geometry with the DMSO ligands situated in the apical and one of the basal positions. These units are linked into chains by SbCl3 molecules acting as Cl-acceptors. Sb(2) forms two chelating chloro-bridges with cis-Cl atoms of one neighbouring complex and a third chloro-bridge with the remaining Cl of the symmetry-related second neighbour. The resulting Cl6-geometry around Sb(2) is distorted octahedral. 1b consists of square-pyramidal molecules, in which the DMSO ligands occupy two basal cis-positions; the monomeric units are loosely linked through the apical Cl atoms to form a chain of octahedra sharing trans-vertices. The asymmetric unit of 2b is a square pyramid with trans-basal DMSO ligands. The pyramids are connected by symmetry-equivalent basal chlorine atoms into chains of octahedra sharing cis-vertices. 1a displays remarkably short Sb-O dative bonds (204.7/212.9 pm); the M-O bond distances of 1b and 2b are 223.0/234.6 and 234.5/238.7 pm, respectively.