How to capture C₂O₂: structures and bonding of neutral and charged complexes [(NHC)-C₂O₂-(NHC)]^q (NHC = N-heterocyclic carbene; q = 0, 1+, 2+)

We present the results of DFT calculations and a thorough bonding analysis of the neutral and charged complexes of the elusive C₂O₂ species stabilized by two NHC ligands. It is shown that the thermodynamic stability of the neutral complex [(NHC)-C₂O₂-(NHC)] is due to the low-lying triplet state of [NHC-CO] (T), which is only 3.2 kcal mol⁻¹ higher in energy than the singlet state [NHC-CO] (S), while the triplet state of CO is 131.9 kcal mol⁻¹ above the singlet. The much lower S/T gap of [NHC-CO] than in CO comes from the charge donation of NHC into the degenerate π* LUMO of CO and the concomitant mixing of the LUMO of NHC with the degenerate π* LUMO of CO, which strongly lowers the energy difference between HOMO and LUMO in the complex. The energy gain resulting from the formation of the C 00000000 00000000 00000000 00000000 11111111 00000000 11111111 00000000 00000000 00000000 C double bond compensates the singlet-triplet gap and the thermodynamic instability of the fragments [NHC-CO] (S). The dissociation of neutral [(NHC)-C₂O₂-(NHC)] to 2NHC and 2CO molecules is calculated to be endothermic by D_o = 78.2 kcal mol⁻¹. The bonding analysis indicates that the neutral and the charged molecules [(NHC)-C₂O₂-(NHC)]^q have a central unit with C-C single bonds, where a combination of electron sharing and s dative interactions leads to very strong carbon-carbon bonds complemented by minor π-donation, which make all systems stable with respect to dissociation reactions.