Dispersion Forces and Counterintuitive Steric Effects in Main Group Molecules: Heavier Group 14 (Si-Pb) Dichalcogenolate Carbene Analogues with Sub-90° Interligand Bond Angles

The synthesis and spectroscopic and structural characterization of an extensive series of acyclic, monomeric tetrylene dichalcogenolates of formula M(ChAr)2 (M = Si, Ge, Sn, Pb; Ch = O, S, or Se; Ar = bulky m-terphenyl ligand, including two new acyclic silylenes) are described. They were found to possess several unusual features—the most notable of which is their strong tendency to display acute interligand, Ch–M–Ch, bond angles that are often well below 90°. Furthermore, and contrary to normal steric expectations, the interligand angles were found to become narrower as the size of the ligand was increased. Experimental and structural data in conjunction with high-level DFT calculations, including corrections for dispersion effects, led to the conclusion that dispersion forces play an important role in stabilizing their acute interligand angles.