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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 Do = 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. The central C2O2 fragment carries a large negative partial charge in the neutral and singly charged compounds [(NHC)-C2O2-(NHC)]0,1+, while it is neutral in the dication [(NHC)-C2O2-(NHC)]2+.