Synthesis and quantum crystallographic evaluation of WYLID: YLID's red rival

The synthesis and characterization of WYLID [2-(di-methyl-λ⁴-sulfane-ylidene)-[1,2'-bi-indenyl-idene]-1',3,3'(2<i>H</i>)-trione], a side product that appears during the synthesis of YLID, is presented in this study. Quantum crystallographic insights into the bonding situation in WYLID are provided on the basis of Hirshfeld atom refinement (HAR), multipolar modelling and X-ray restrained wavefunction fitting. We compare these results with theoretical calculations on a gas-phase optimized density functional theory and a Hartree-Fock calculation based on the geometry obtained by HAR. The S-C bond in WYLID is best described as an ylid-type bond and the SMe₂ fragment does not affect the nearest C-O carbonyl/enolate equilibrium. This work may contribute to an ongoing debate in the quantum crystallography community about the bonding situation in YLID and provides a routine for distinguishing between carbonyls and enolates.