[(thf)Li21{H2CS(NtBu)2}]2:: Synthesis, polymorphism, and experimental charge density to elucidate the bonding properties of a lithium sulfur ylide

Sulfur ylides (R2S+--CR2) are widely used in organic synthesis for stereoselective epoxidations, cyclopropane formations, and ring expansion reactions. Nevertheless, their electronic properties are still under debate, because their ylenic textbook formulation (R2S=CR2) contradicts the reactivity. In order to elucidate the electronic situation in a sulfur ylide, we present an experimental charge density study via multipole refinement and subsequent topological analysis based on high-resolution X-ray data of [(thf)Li-2{H2CS((NBu)-Bu-t)(2)}](2) (1). The title compound is of special interest, since additionally the formal hypervalency can be investigated along with the controversial interaction between a carbanion with a Li-3 triangle. As a prerequisite for these studies, the polymorphism and synthesis of I were investigated. The findings clearly support the ylidic, non-hypervalent description of the molecule. The ylidic carbanion was determined to be 6-fold coordinated with three single bonds to the sulfur atom and both hydrogen atoms and three closed-shell interactions to the lithium atoms of the Li-3 triangle.