Different Donor-Acceptor Interactions of Carbene Ligands in Heteroleptic Divalent Group 14 Compounds, LEL′ (E = C-Sn; L = N-Heterocyclic Carbene; L′ = Cyclic Alkyl(Amino) Carbene)

The electronic structure and reactivity of heteroleptic divalent group 14 compounds, 1E (E = C-Sn) with NHC and cAAC ligands have been studied at the BP86/TZ2P level of theory and compared with homoleptic group 14 compounds. The EDA-NOCV (energy decomposition analysis-natural orbitals for chemical valence) analysis indicates that the interaction between the two carbene ligands and the central C-atom in 1C can be best represented as one 3c-2e electron sharing sigma-bond and one 3c-2e donor-acceptor sigma-bond. There exists an electron sharing interaction between the pi-type orbital on the central C-atom and the C-N pi orbital of cAAC and pi-back-donation from the sigma-type lone pair on the central C-atom to the pi*-MO of NHC. This bonding description is equivalent to the localized bonding representation, where the central C-atom forms two electron sharing bonds and two donor-acceptor bonds with cAAC and NHC ligands. However, the bonding between the carbene ligands and the heavier group 14 element can be best represented as two 2c-2e donor-acceptor sigma-bonds and pi-back-donation from group 14 element to C-N pi* orbital of cAAC. This bonding description is well supported by the geometrical and Natural Bond Orbital (NBO) analyses. Hence, 1C can be best described as a carbene and the heavier analogues can be best described as tetrylones. However, the high first (287.6-274.3 kcal mol(-1)) and second proton affinities (162.0-158.5 kcal mol(-1)) suggest that 1E (E=C-Sn) behave as tetrylones.