(Carbene) → SO ₂ complexes: ligand-stabilised carbocyclic carbenes ( <i>ls</i> CCCs) are better than N-heterocyclic carbenes

Cyclic carbenes can form coordination complexes with sulfur dioxide (SO₂). Two N-heterocyclic carbene (NHC) → SO₂ coordination complexes were reported along with their crystal structures. However, computational analyses of the nature and strength of coordination bonds provided conflicting indications. Thus, it was essential to identify the properties that enhance the bond strength of carbene → SO₂. In this work, density functional theory (DFT) studies were carried out to estimate the electronic, geometric and energetic parameters of several (NHC) → SO₂ complexes. This study was extended to include carbocyclic carbene (CCC) → SO₂, remote N-heterocyclic carbene (<i>r</i>NHC) → SO₂, abnormal N-heterocyclic carbene (<i>a</i>NHC) → SO₂, cyclic alkyl amino carbene (CAAC) → SO₂, and amido carbene (<i>am</i>NHC) → SO₂ complexes. Results revealed a set of ligand-stabilised carbocyclic carbenes (<i>ls</i>CCCs), which could stabilise the donor → acceptor bonds between cyclic carbenes and SO₂.