**3. Results**

The geometry of the model system, taken directly from the X-ray coordinates, is exhibited in Figure 2a, which surrounds the PdCl4 <sup>2</sup><sup>−</sup> dimer by four counterion ligands. The reader should be aware that a finite excerpt from a full crystal structure is considered here. There are a multitude of H-bonds connecting these counterions to the dianions. To avoid overcomplication of the figure, only very short ones, with R(H···Cl) less than 2.4 Å, are shown explicitly by the broken blue lines. Figure 2b focuses on the dianion dimer itself, showing clearly that the Cl of the top unit approaches the Pd of the lower to within 3.217 Å.

**Figure 2.** (**a**) Geometry of the PdCl4 <sup>2</sup><sup>−</sup> dimer, surrounded by four counterions, with coordinates taken directly from X-ray structures; H atom positions optimized. (**b**) Detailed structure of [PdCl4 <sup>2</sup>−]2. Distances in Å, angles in degs.
