Neutral carbene analogues of group 13 elements: the dimerization reaction to a biradicaloid.
The dimerization reactions of the neutral carbene analogues with the group 13 elements boron, aluminum, gallium, and indium are studied. Besides boron, all monomeric species possess singlet ground states. For Al, bulky substituted cases were investigated; they reveal no essential changes in the singlet-triplet energy separations compared with the parent species. The dimerization energies increase with an increase in the bulk of the substituents; this is a consequence of an enhancement of van der Waals forces for association. The latter is opposed by entropic forces, which facilitate dissociation. An equilibrium between monomeric and dimeric structures is predicted because of enthalpy versus entropy control. The low-temperature domain association should prevail in the formation of a dimer with Al (Ga) within the formal oxidation state I+. The Al-Al bond refers to a chelated biradicaloid species with an energetically low-lying triplet state. It emerges from the metal-metal contacts in the dimer. The biradical character of the dimer decreases in the order E = Al ≫ Ga > In. The carbene analogue of In forms upon dimerization of only weak coordinative metal-metal interactions.