Comparison of the Coordination of B 12 F 12 2- , B 12 Cl 12 2- , and B 12 H 12 2- to Na + in the Solid State: Crystal Structures and Thermal Behavior of Na 2 (B 12 F 12 ), Na 2 (H 2 O) 4 (B 12 F 12 ), Na 2 (B 12 Cl 12 ), and Na 2 (H 2 O) 6 (B 12 Cl 12 )

The synthesis of high-purity Na B F and the crystal structures of Na (B F ) (5 K neutron powder diffraction (NPD)), Na (H O) (B F ) (120 K single-crystal X-ray diffraction (SC-XRD)), Na (B Cl ) (5 and 295 K NPD), and Na (H O) (B Cl ) (100 K SC-XRD) are reported. The compound Na (H O) (B F ) contains {[(Na(μ-H O) Na(μ-H O) )] } infinite chains; the compound Na (H O) (B Cl ) contains discrete [(H O) Na(μ-H O) Na(H O) ] cations with OH···O hydrogen bonds linking the terminal H O ligands. The structures of the two hydrates and the previously published structure of Na (H O) (B H ) are analyzed with respect to the relative coordinating ability of B F , B H , and B Cl toward Na ions in the solid state (i.e., the relative ability of these anions to satisfy the valence of Na ). All three hydrated structures have distorted octahedral NaX (H O) coordination spheres (X = F, H, Cl). The sums of the four Na-O bond valence contributions are 71, 75, and 89% of the total bond valences for the X = F, H, and Cl hydrated compounds, respectively, demonstrating that the relative coordinating ability by this criterion is B Cl ≪ B H < B F . Differential scanning calorimetry experiments demonstrate that Na (B F ) undergoes a reversible, presumably order-disorder, phase transition at ca. 560 K (287 °C), between the 529 and 730 K transition temperatures previously reported for Na (B H ) and Na (B Cl ), respectively. Thermogravimetric analysis demonstrates that Na (H O) (B F ) and Na (H O) (B Cl ) undergo partial dehydration at 25 °C to Na (H O) (B F ) and Na (H O) (B Cl ) in ca. 30 min and 2 h, respectively, and essentially complete dehydration to Na (B F ) and Na (B Cl ) within minutes at 150 and 75 °C, respectively (the remaining trace amounts of H O, if any, were not quantified). The changes in structure upon dehydration and the different vapor pressures of H O needed to fully hydrate the respective Na (B X ) compounds provide additional evidence that B Cl is more weakly coordinating than B F to Na in the solid state. Taken together, the results suggest that the anhydrous, halogenated closo-borane compounds Na (B F ) and Na (B Cl ), in appropriately modified forms, may be viable component materials for fast-ion-conducting solid electrolytes in future energy-storage devices.