Olefin metathesis has become a device of prominent importance for C-C double bond formation thus earning an outstanding position in the organic synthesis toolbox. Especially well-defined ruthenium-based alkylidene complexes are widely used as catalysts due to their stability towards air and moisture. The discovery of N-heterocyclic carbenes (NHCs) has made a great impact in the field, giving birth to even more stable and efficient second generation catalysts (e.g. Ru1 and Ru2, Figure ). Despite the incredible progress made in the discipline, there are still many hurdles to overcome, one of which is the ineffectiveness of these popular, general-purpose ruthenium catalysts in the formation of tetrasubstituted or crowded C-C double bonds (Figure ). Figure . Selected general-purpose catalysts (Ru1, Ru2) versus sterically reduced ones (Ru3-Ru8) designed for the formation of tetrasubstituted C-C double bonds, and associated decomposition products (Ru9, Ru10). In the inset is shown an example of a challenging metathesis reaction. In 2007, Grubbs and Schrodi reported a practical solution to this essential problem by developing a series of ruthenium catalysts bearing NHC ligands with sterically less hindered, mono-ortho substituted N-aryl groups (Ru3 and Ru4, Figure ). The proposed explanation for their higher activity is fairly straightforward and widely accepted-the smaller demand of the "chopped" NHC ligand allows for more facile coordination of sterically crowded olefins to the ruthenium metal centre. At about the same time, other structures were developed (e.g. Ru5 -Ru7, Figure ) based on the same concept. , ,For other examples of Ru-based catalysts specially-designed for sterically-demanding substrates, please see: Although these new catalysts are more efficient in the formation of crowded and tetrasubstituted olefins than popular general purpose complexes, such as Ru1 and Ru2, it was quickly observed that they strongly suffer from decreased stability and increased sensitivity to air and other poisons. Blechert et al. investigated the mechanism of deactivation of Ru6, which consisted of sequential pericyclic cyclisation, oxidation, and elimination steps yielding the metathesis-inactive product Ru9 (Figure ). Particularly, when both arms of the NHC ligand are sterically reduced, the resulting catalyst (e.g. Ru7) is extremely prone to deactivation, following pathways including C-H activation in the ortho N-phenyl position, which after insertion in alkylidene gives the permanently deactivated complex Ru10 (Figure ). Figure 2. Selected approaches to stabilize small-NHC ruthenium catalysts, A) by using stabilizing ligands, B, C) by restricting aryl ring rotation. See, Refs.: [13] and [14].