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 carbodi-carbenes 1 and 2, for the bent allenes 3a, 3b, 4a, and 4b, and for the carbo-carbenephosphoranes 7a, 7b, and 7c. The last-named molecules have very large first and large second proton affinities. They also bind two BH3 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 N-2 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 CL2 are p 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.