[(thf)Li2{H2CS(NtBu)2}]2: Synthesis, Polymorphism, and Experimental Charge Density to Elucidate the Bonding Properties of a Lithium Sulfur Ylide

Sulfur ylides (R 2 S + − − CR 2 ) 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 (R 2 S═CR 2 ) 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 {H 2 CS(N t Bu) 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 1 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.