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In the trigonal-bipyramidal species H2CSF4 (1) the H atoms are located in the plane of the axial F atoms (low-temperature crystal structure analysis, electron diffraction). The PE spectrum confirms that the resonance structure having a double bond has greater weight than in other ylides. (1) is exceptionally stable thermally, but highly reactive (addition to the polar CS bond, CH2 transfer).
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Bock et al. (1979) studied this question.
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