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 charge- and energy-decomposition analysis of the electronic structure of compounds 110 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” 3 a, 3 b, 4 a, and 4 b, and for the carbocarbenephosphoranes 7 a, 7 b, and 7 c. 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 810 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 110 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 π donors and thus may serve as double Lewis bases, while divalent carbon(II) compounds are π acceptors. The theoretical results point toward new directions for experimental research in the field of low-coordinate carbon compounds.