*Article* **The Role of Hydrogen Bonds in Interactions between [PdCl4] 2***−* **Dianions in Crystal**

**Rafał Wysoki ´nski 1,\*, Wiktor Zierkiewicz 1,\*, Mariusz Michalczyk 1,\*, Thierry Maris <sup>2</sup> and Steve Scheiner <sup>3</sup>**


**Abstract:** [PdCl4] <sup>2</sup><sup>−</sup> dianions are oriented within a crystal in such a way that a Cl of one unit approaches the Pd of another from directly above. Quantum calculations find this interaction to be highly repulsive with a large positive interaction energy. The placement of neutral ligands in their vicinity reduces the repulsion, but the interaction remains highly endothermic. When the ligands acquire a unit positive charge, the electrostatic component and the full interaction energy become quite negative, signalling an exothermic association. Raising the charge on these counterions to +2 has little further stabilizing effect, and in fact reduces the electrostatic attraction. The ability of the counterions to promote the interaction is attributed in part to the H-bonds which they form with both dianions, acting as a sort of glue.

**Keywords:** π-hole; counterions; molecular electrostatic potential; AIM; energy decomposition
