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(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)(2)C = 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 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(D-1) donor-acceptor bonds are roughly twice as strong as the respective L -> BH3 bond.