Carbodicarbenes and related divalent carbon(0) compounds

Quantum-chemical calculations using DFT and ab initio methods have been carried out for fourteen divalent carbon(0) compounds (carbones), in which the bonding situation at the two-coordinate carbon atom can be described in terms of donor-acceptor interactions L→C←L. The chargeand energy-decomposition analysis of the electronic structure of compounds 1-10 reveals divalent carbon(0) character in different degrees for all molecules. Carbone-type bonding L→C←L is particularly strong for the carbodicarbenes 1 and 2, for the "bent allenes" 3a, 3b, 4a, and 4b, and for the carbocarbenephosphoranes 7a, 7b, and 7c. The last-named molecules have very large first and large second proton affinities. They also bind two BH ₃ ligands with very high bond energies, which are large enough that the bis-adducts should be isolable in a condensed phase. The second proton affinities of the complexes 5, 6, and 8-10 bearing CO or N₂ as ligand are significantly lower than those of the other molecules. However, they give stable complexes with two BH₃ ligands and thus are twofold Lewis bases. The calculated data thus identify 1-10 as carbones L→C←L in which the carbon atom has two electron pairs. The chemistry of carbones is different from that of carbenes because divalent carbon(0) compounds CL ₂ are π donors and thus may serve as double Lewis bases, while divalent carbon(II) compounds are p acceptors. The theoretical results point toward new directions for experimental research in the field of low-coordinate carbon compounds.