Explaining the Binding Puzzle of N‐Heterocyclic Carbenes on Oxide Surfaces by Means of Density Functional Theory Calculations

N-Heterocyclic carbenes (NHCs) have been successfully employed as ligands for the modification of metallic surfaces. Their adsorption on oxide surfaces remains less explored. Here we show, by means of density functional theory calculations, how five model NHCs bind covalently to different oxide surfaces, spanning from ionic (MgO) or covalent (SiO2) large-gap oxides to small-gap TiO2 and Cu2O. A complex trend emerges, where NHCs are adsorbed only on positively charged surface sites on MgO and SiO2, while both metal cations and oxygen anions can coordinate the molecules on Cu2O and TiO2. On Cu2O, however, bonding on Cu sites is more favorable, while the opposite is found for TiO2. The key aspect is that the bond to metal cations (or silicon) involves little charge transfer, while the bond to O implies a substantial electron donation to the oxide surface. The reducibility of the oxide and the alignment between its band edges and the NHC frontier orbitals are thus the determining factors for understanding this behavior.