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

Quantum‐chemical calculations with DFT (BP86) and ab initio methods [MP2, SCS‐MP2, CCSD(T)] have been carried out for the molecules C(PH 3 ) 2 ( 1 ), C(PMe 3 ) 2 ( 2 ), C(PPh 3 ) 2 ( 3 ), C(PPh 3 )(CO) ( 4 ), C(CO) 2 ( 5 ), C(NHC H ) 2 ( 6 ), C(NHC Me ) 2 ( 7 ) (Me 2 N) 2 CCC(NMe 2 ) 2 ( 8 ), and NHC ( 9 ), where NHC=N‐heterocyclic carbene and NHC Me = 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 L 2 C 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=PR 3 , 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 CCC 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 lone‐pair character, which suggests that even the quasi‐linear tetraaminoallene 8 is a “masked” divalent carbon(0) compound. This explains the very shallow bending potential of 8 . The same conclusion is drawn for phosphoranylketene 4 and for carbon suboxide ( 5 ), which according to the bonding analysis have hidden double‐lone‐pair character. The AIM analysis and the EDA calculations support the assignment of carbodiphosphoranes as divalent carbon(0) compounds, while NHC 9 is characterized as a divalent carbon(II) compound. The L→C( 1 D) donor–acceptor bonds are roughly twice as strong as the respective L→BH 3 bond.