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,), (1), C(PMe₃)₂ (2), C-(PPh₃)₂ (3), C(PPh₃)(CO) (4), C(CO)₂ (5), C(NHC_H) ₂ (6), C(NHC_Me)₂ (7) (Me₂N) ₂C=C=C(NMe₂)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 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 orbitais L→C←L. This holds particularly for the carbodiphosphoranes 1-3 where L = PR₃, which therefore are classified as divalent carbon(O) 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(¹D) donor-acceptor bonds are roughly twice as strong as the respective L→ BH₃ bond.