Unravelling the Dramatic Electrostructural Differences Between N‑Heterocyclic Carbene- and Cyclic (Alkyl)(amino)carbene-Stabilized Low-Valent Main Group Species

Cyclic (alkyl)­(amino)­carbenes (CAACs) and N-heterocyclic carbenes (NHCs) are widely used as stabilizing ligands in transition metal and main group element chemistry. Variations in their stabilizing properties have been cursorily explained in the literature by the greater π-donating and σ-accepting properties of CAACs relative to NHCs and their differing steric demands; however, a more precise understanding, in particular a disentanglement of steric and electronic effects, is lacking. The recently reported compounds (E)­(L)­BB­(L)­(E) (L = NHC (I)/CAAC (II) and E = SPh) present a unique opportunity to investigate the differences between NHC and CAAC donors, as both forms are stable but differ considerably in their geometrical and electronic properties. The NHC systems possess a singlet ground state with a planar central SBBS unit, while their CAAC counterparts show a triplet ground state with a twisted SBBS unit. Steric effects were found to be important in this case; however, it remained unclear how the different forms of twisting in I and II depend on the interplay of steric and electronic effects. In the present work we disentangle both effects. Our investigations explain all of these effects by MO considerations and show that for this kind of system the size of the singlet–triplet gaps are the key determinants of the differences. The different sizes of the S–T gaps result from variations in the antibonding effects within the highest occupied (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Our explanation seems to contradict the general scientific consensus about variations in the HOMO and LUMO of these two classes of cyclic carbenes; however, comparisons to the Kekulé biradicaloids recently presented by Bertrand and co-workers indicate the generality of our approach.