Unusual structural features of a siloxane |
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Affiliation: | 1. Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, 61-614 Poznań, Poland;2. Centre for Advanced Technologies, Adam Mickiewicz University in Poznań, Umultowska 89c, 61-614 Poznań, Poland;3. Adam Mickiewicz University Foundation, Rubież 46, 61-612 Poznań, Poland |
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Abstract: | Crystals of the R, S diastereoisomer of [Cp(CO)2-FeSiCH3F]2O are monoclinic, space group P21/c (No. 14), with a = 846.0(3) [836.4(1)], b = 768.0(3) [757.1(1)], c = 1548.5(4) [1522.3(2)] pro, β = 97.34(3)° [97.47(3)°] at 300 K [120 K] with Z = 2. Even at 120 K the SiOSi fragment is found to be strictly linear due to crystallographically imposed symmetry. To explain the unusual electron distribution derived from the X-ray data collected, several types of possible disorders are discussed, none of which leads to a satisfying explanation. Retaining the Ci symmetry (linear SiOSi fragment in the final model) the important bond lengths are FeSi 226.7(1) [226.5(1)] pm, SiF 160.9(2) [161.8(2)] pm, SiO 160.3(1) [161.1(1)] pm, SiC 185.0(3) [185.6(3)] pm. The electronic features of this compound were probed via molecular orbital calculations of the extended Hückel type. It was found that the lone pairs on the siloxane oxygen were tipped away from cylindrical symmetry. The tipping was directed toward the fluorine substituents on the silicon atoms and away from the CpFe(CO)2 units. A pertubational approach was utilized to rationalize this effect. |
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