The crystal structures of NSF 3 and (NSF 2N(CH 3)CH 2–) 2: How short is the ‘Crystallographic’ N [tbnd]S triple bond?

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The synthesis of the first unequivocally characterised bis(difluorothiazyne), [N SF 2N(CH 3)CH 2–] 2 is reported. The crystal structures of this and NSF 3 are also reported. NSF 3 has the same geometrical parameters, within error, as it does in the gas phase. PIXEL calculations show that the principal interactions in its crystal structure are S N⋯S N dipolar contacts, which form chains with S⋯N = 3.533(2) Å. These contacts are reminiscent of those observed in the crystal structures of ketones. The exchange of a fluorine by a dialkylamino group has almost no influence on the NS bond distance while the SF bonds are significantly elongated. This behaviour is explained by negative hyperconjugation and confirmed by experimental data (as far as available) and quantum chemical calculations for N SF n (NMe 2) 3− n and N SF n Ph 3− n ( n = 1–3).