Abstract
The results of a detailed analysis of typical situations in crystals containing trigonal-pyramidal YEL3ε species are used to argue that, perse, the lone electron pair (E, LP) on the Y atom determines neither the crystal structure nor the orientation of the YEL3ε units in the crystal. Considered as a space-filling entity, the LP has practically no effective volume of its own. Whatever space-filling or orienting properties may be associated with the presence of the LP, they are not autonomous; they arise from the intrinsic stereochemical involvement of the LP in the geometry of and the charge distribution in the free YEL3ε ion or molecule and hence from the consequences of this geometry for crystal packing. Abinitio (6-31G*) calculations show that the electron density in the region of the LP in an ion is only slightly perturbed by the charge on the counter ion at distances observed in crystals. There is considerable similarity in the electron density distribution in the LP region of Y between free anionic (SO32−) and free cationic (Me3S+) species. The problem of the so-called stereochemical inertness of the LP in the 14-electron octahedral YEX62− ions in crystals is examined in detail. It is concluded that the crystallographic Oh symmetry of the ion in cubic crystals is the result of dynamic averaging of possible orientations of the inherently noncubic YX62− octahedron (cf. the abinitio results for SeCl62−). Such averaging also appears to be responsible for the stereochemical "activation" or "inactivation" of the LP in thermally induced crystallographic transitions in the AYX3 halide perovskites and related crystals. To provide a firmer basis for the discussion of the LP, the crystal structures of the five title compounds have been determined. The crystal chemistry of these and related compounds is discussed. Keywords: abinitio calculations, Me3S+ and SeCl62−; group V trihalides; lone electron pair; trialkylsulfonium salts; VSEPR theory.
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