Crystal and molecular structures of disilyl sulphide (at 120 K) and of disilyl selenide (at 125 K) and comparison with crystalline disilyl oxide

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Abstract
Crystals of SiH3–S–SiH3(at 120 K) are orthorhombic, space group Pbcn, with a= 8.14, b= 14.76, c= 8.68 Å[estimated standard deviations (e.s.d.s) 0.3% assumed] and Z= 8. Crystals of SiH3–Se–SiH3(at 125 K) are tetragonal, space group P43212, with a= 8.36 Å(e.s.d. 0.4% assumed), c= 15.4 Å(e.s.d. 0.6% assumed), and Z= 8. Crystals were grown ‘in situ’ on a Weissenberg goniometer fitted with low-temperature equipment. Using photographic (microdensitometer) intensities the structures have been refined to R= 0.038 over 410 reflexions (SiH3–S–SiH3), and R= 0.055 over 263 reflexions (SiH3–Se–SiH3). The heavy-atom geometries are 〈Si–S〉= 2.142(7)Å with Si–S–Si = 98.4(3)°, and 〈Si–Se〉= 2.27(2)Å with Si–Se–Si = 95.7(5)°. There is specific molecular alignment in both crystal structures giving rise to Si ⋯ S (or Se) intermolecular interactions; the lengths of these contacts are 0.35–0.4 Å less than the sum of van der Waals radii. In both structures each Si atom has 4 + 1 co-ordination while the S (or Se) atoms have 2 + 2 co-ordination; this is in contrast to crystalline SiH3–O–SiH3 where only one Si atom is involved in an intermolecular contact to oxygen.

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