The Pseudo-Symmetric Structure of Pb(SPh)₂

The crystal structure of Pb(SC₆H₅)2 is pseudo-C-centred orthorhombic, a = 54.06 (1), b = 11.468 (1), c = 7.4387 (8) Å, = = = 90°, Z = 16, and may be described as a partial ordering of a 1:1 disordered parent structure of symmetry Pmcn, Z = 4 (a' = a/2, b' = b/2, c' = c), in which the mirror imposes a 1:1 disorder on two-dimensionally polymeric layers perpendicular to a*. An ideally ordered structure has monoclinic space group C112₁/d (P2₁/c using an alternative axis system b, c, [a + b]/2), but may also be described as two inversion-related substructures of Cmc2₁ pseudo- symmetry, where the b-glide planes of one substructure coincide with the mirror planes of the other and vice versa. Moving one substructure by b/2 relative to the other creates a different orientation of the structure. The crystal studied showed a partial disorder of each substructure relative to origins b/2 apart [0.964 (4):0.036 for one substructure and 0.584 (3):0.416 for the other]. This lowers the symmetry of the average structure to C112₁ with intensities realistically described as K²[(1-)|F(hkl)|² + |F(hkl)|²], where K² for h odd, k odd reflections is 0.444 (7) of the value for h even, k even reflections, is 0.325 (5) and F(hkl) is the structure factor for an ideally ordered structure. Final values for R of 0.046 and 0.090 were obtained for the 844 h even, k even and 687 h odd, k odd reflections with I(h) > 3(I(h)) used in refinement. A bond-valence interpretation of the bonding within the polymeric layer structure is given.

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