Divalent Carbon(0) Chemistry, Part 1: Parent Compounds

AbstractQuantum‐chemical calculations with DFT (BP86) and ab initio methods [MP2, SCS‐MP2, CCSD(T)] have been carried out for the molecules C(PH3)2 (1), C(PMe3)2 (2), C(PPh3)2 (3), C(PPh3)(CO) (4), C(CO)2 (5), C(NHCH)2 (6), C(NHCMe)2 (7) (Me2N)2CCC(NMe2)2 (8), and NHC (9), where NHC=N‐heterocyclic carbene and NHCMe=N‐methyl‐substituted NHC. The electronic structure in 1–9 was analyzed with charge‐ and energy‐partitioning methods. The results show that the bonding situations in L2C compounds 1–8 can be interpreted in terms of donor–acceptor interactions between closed‐shell ligands L and a carbon atom which has two lone‐pair orbitals L→C←L. This holds particularly for the carbodiphosphoranes 1–3 where L=PR3, which therefore are classified as divalent carbon(0) compounds. The NBO analysis suggests that the best Lewis structures for the carbodicarbenes 6 and 7 where L is a NHC ligand have CCC double bonds as in the tetraaminoallene 8. However, the Lewis structures of 6–8, in which two lone‐pair orbitals at the central carbon atom are enforced, have only a slightly higher residual density. Visual inspection of the frontier orbitals of the latter species reveals their pronounced lon