**4. Conclusions**

In this article, forty dimers formed by imidazol-2-ylidene or its derivatives, in which the hydrogen atoms of both N-H bonds were replaced by important and popular substituents of increasing complexity (methyl = Me, *iso*-propyl = *<sup>i</sup>* Pr, *tert*-butyl = *<sup>t</sup>* Bu, phenyl = Ph, mesityl = Mes, 2,6-diisopropylphenyl = Dipp, 1-adamantyl = Ad), and five fundamental proton donor (HD) molecules (HF, HCN, H2O, MeOH, NH3), have been studied. Although the most important goal was to describe the hydrogen bond formed by the carbene carbon atom, C··· H-D, an equally important issue, which is, in the author's opinion, insufficiently described in the literature, was the possibility of the formation of various accompanying interactions and their influence on the structure of the considered dimers. Despite different interaction abilities represented by carbenes and especially HD molecules, the following general trends have been found, in addition to many specific results:


The article clearly shows that various types of secondary interactions, thus often omitted in the analysis, can have a significant impact on the structure of a molecular system and its strength. The results should improve our understanding of carbene chemistry and the role of intermolecular interactions.

**Funding:** This research received no external funding.

**Data Availability Statement:** Data available from the author on reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.
