**4. Conclusions**

A theoretical study and CSD search of the different effects provoked by substituents and ligands upon complexation with dinuclear Ag(I) pyrazolates has been carried out and the structural, energetic, electron density and magnetic features analyzed.

The CSD search shows a grea<sup>t</sup> variability of the Ag–Ag distance in the crystal structures. These results have been rationalized based on the number of ligand interactions with the Ag atoms and the substituents of the pyrazole ring.

In the isolated (PzAg)2 system, it was observed that for Ag–Ag the longer the distance, the lesser the stability of the unsubstituted complex with no ligands. This decrease in the stability is somehow compensated by the ligands upon complexation.

Furthermore, complexation with ligands through the Ag atoms increases the intramolecular distance Ag–Ag. In fact, considering the PH3 ligand, the increase of the Ag–Ag distance was found moderate when going from no ligand to one ligand. However, when two simultaneous PH3 are interacting, the Ag–Ag distance increases dramatically. Nevertheless, when four PH3 are considered, the increase is again moderate.

In terms of the QTAIM analysis, it is noteworthy the presence of a BCP between both Ag atoms among all the systems with Ag–Ag distances shorter than 3.3 Å.

Finally, regarding the aromatic/non-aromatic properties, six-membered rings containing the Ag–Ag motif show negative NICS values but those reveal a non-aromatic character mainly affected by the proximity of the Ag nuclei. This was confirmed by the relationship found between the NICS values and the Ag–NICS distance. On the other hand, pyrazole rings maintain their aromatic behaviour with slight changes.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4352/10/2/137/s1, Table S1: Two simplified views of the structures found in the CSD search, Table S2. Effect of the Ag(I)–Ag(I) distance (Å) on the relative energy (kJ·mol−1) of the (PzAg)2 system, Table S3: Cartesian coordinates (Å) of the optimized systems at MP2/jul-cc-pVDZ/jul-cc-pVDZ-PP level, Figure S1: Relative energy vs. Ag–Ag distance in the (PzAg)2 system, Figure. S2: De (kJ·mol−1) vs. Ag–Ag dist. (Å) in the (PzAg)2L2 complexes, Figure S3: De (kJ·mol−1) vs. Ag–Ag dist. (Å) in the (R-pzAg)2(PH3)2 complexes, Table S4: Distance (Å) and electron density properties (au) of the Ag–Ag BCPs, Table S5: Distance (Å) and electron density properties (au) of the Ag–L BCPs, Figure S4: 109Ag chemical shielding vs. Ag–Ag distance in the (PzAg)2L2 complexes. Table S6. NICS values (ppm) from 0.0 to 2.0 Å of the center of the 6-membered ring in (PzAg)2.

**Author Contributions:** Conceptualization and project design, I.A., J.E.; resources, I.A., C.T., G.S.-S, data curation, I.A, J.E, C.T., G.S.-S., writing—original draft preparation, I.A, J.E, C.T., G.S.-S.; writing—review and editing, I.A, J.E, C.T., G.S.-S.; funding acquisition, I.A. C.T.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was carried out with financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades (Projects PGC2018-094644-B-C2) and Dirección General de Investigación e Innovación de la Comunidad de Madrid (PS2018/EMT-4329 AIRTEC-CM), and Science Foundation of Ireland (SFI), gran<sup>t</sup> number 18/SIRG/5517.

**Acknowledgments:** We are grateful to the Irish Centre for High-End Computing (ICHEC) and CTI(CSIC) for the provision of computational facilities. We are also grateful to Dr Lee O'Riordan for his continuous support and help.

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