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.