An isolable heterocyclic silylene (4) with two different π-donating substituents, namely a classical amino group and a more electropositive and stronger carbon-based π-donating phosphonium ylide, was synthesized and fully characterized. The combination of these two different π-donating substituents confers high thermal stability and an unusual nucleophilic character on silylene 4. Therefore, silylene 4 behaves as a strong donor ligand toward transition metals with a donating character comparable to N-heterocyclic carbenes, in contrast to classical N-heterocyclic silylenes, which generally present a weak donating character.

A borylidene-phosphorane, the lightest phosphoranylidene, which is stabilized by an N-heterocyclic carbene ligand, was synthesized and fully characterized. Experimental electron density analysis and DFT calculations indicate an enhanced ylene character (rather than an ylide character) with an exceptionally strong B→P π-back bonding related to the less electronegative boron compared to phosphorus.