Hidden Dicarbene Nature of Acetylenes and Captodative Bonding on Carbon

Abstract

The density functional theory derived molecular electrostatic potential (MESP), 13C NMR chemical shift (δ), bond order, and coordination reactions show that alkynes (RCCR) attain 1,2‐dicarbene nature during CCR angle bending. Alkyne carbon atoms of bent structures exhibit MESP features unique to lone‐pair bearing atoms, δ around 200 ppm typical for carbene centers and large reduction in CC triple bond character. Lone pair bearing atoms of R substituents enhance the carbene character. The bent alkynes can be trapped with Lewis acids (BH3, BF3, AlF3 and AlCl3) as the lone pairs developed on carbon centers provide strong donor type dative bonding. The dative bond gives a formal valence electron count six on carbon and suggests the for‐mation of acceptor type dative bonding to carbon from Lewis base (NH3). Reaction of alkynes with (Lewis acid)←(Lewis base) systems yield (Lewis acid)2←(Alkyne)←(Lewis base)2 complexes which are exothermic and exergonic for many cases. These complexes are examples of captodative carbon(II) compounds.