**4. Conclusions**

In this featured review, we highlighted the importance and nature of the modes of nitrogen-centered pnictogen bonding (or, simply, the nitrogen bond) observed in many crystals deposited in the CSD and ICSD. Several of them have been known since the last century, even though the term "pnictogen bond", or even "nitrogen bond" was not coined during that time. They turn out to occur in four different flavors: N(σ)···lone-pair, N( <sup>π</sup>)···lone-pair, N(σ)··· π and N( <sup>π</sup>)··· π(N). The rationalization of these interacting modes was made possible by exploring the MESP of isolated monomers constituting the crystals in several instances, together with chemical intuition. We have shown that the singular occurrence of pnictogen bonding in crystals is very rare, and hence is accompanied or reinforced by other primary/secondary interactions in a grea<sup>t</sup> majority of crystals, such as halogen bonds, hydrogen bonds, and/or π··· π stacking interactions. Whereas there have been few previous studies reported of pnictogen bonding with covalently bonded nitrogen as an electrophilic agen<sup>t</sup> (viz. imide nitrogen [148]), as the nitrogen in the molecular entity is often observed to be negative, this overview highlighted the local modes of intra- and intermolecular bonding interactions in several examples containing nitrogen that has a positive site. This is expected to guide researchers on future studies of nitrogen-centered pnictogen bonding and the design in silico of novel materials. We did not carry out a statistical analysis of structures deposited in the CSD and ICSD of pnictogen bonding to reveal possible ranges of intermolecular distances and angles of the approach of the electrophile on N in molecular entities, since pnictogen bonding is heavily affected by other primary and secondary interactions. As pointed out by a reviewer, even though a single example would be sufficient proof of existence, stronger arguments can be made if such cases are definitely shown not to be outliers. Since no rigorous theoretical first-principles calculations have been performed on most of the systems highlighted in this study, it is expected that the illustrative crystal systems may be used as model systems to investigate the energy stability and other physio-chemical properties (viz. vibrational frequency shift and NMR chemical shifts) to reveal the characteristic features of pnictogen bonds. The attempt will no doubt help provide a reasonable description of the strength of nitrogen bonds in supermolecular entities.

**Author Contributions:** Conceptualization, project design, and project administration, P.R.V.; formal analysis and investigation, P.R.V. and A.V.; supervision, P.R.V.; writing—original draft, P.R.V. and A.V.; writing—review and editing, P.R.V., H.M.M., A.V. and K.Y. All authors have read and agreed to the published version of the manuscript.

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

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** This research did not report any data.

**Acknowledgments:** This work was entirely conducted using the various computation and laboratory facilities provided by the University of Tokyo and the Research Center for Computational Science of the Institute of Molecular Science (Okazaki, Japan). P.R.V. is currently affiliated with the University of the Witwatersrand (SA), and Nagoya University, Aichi 464-0814, Japan; A.V. is currently affiliated with AIST, Tsukuba 305-8560, Japan; K.Y. is currently affiliated with Kyoto University, ESICB, Kyoto, 615-8245, Japan. H.M.M. thanks the National Research Foundation, Pretoria, South Africa, and the University of the Witwatersrand for funding.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had absolutely no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
