119 Sn Mössbauer Spectroscopy A lab-made constant acceleration Halder-type spectrometer operating in transmission geometry was used to carry out the Mössbauer analyses. The spectrometer was equipped with a radioactive source of 119m Sn (370 MBq) embedded in a CaSnO 3 matrix and maintained at room-temperature. Experiments were performed with 50 mg to 70 mg of sample ([Sn] = 5 mg cm -2 to 8 mg cm -2 ) at room temperature (∼ 293 K) and 77 K using a liquid nitrogen bath cryostat. The Mössbauer hyperfine parameters (δ isomer shift, ∆ quadrupole splitting, Γ signal linewidth, G 11 Goldanskii-Karyagin factor and relative areas) were refined using the WinNormos software [97]. Isomer-shift values are reported relative to CaSnO 3 at room temperature. 19 F Solid-State NMR Quantitative 19 F Magic Angle Spinning (MAS) NMR spectra were recorded on Bruker Avance III spectrometers operating at B 0 = 7.0 T ( 19 F Larmor frequency of 282.4 MHz), using a 1.3 mm CP-MAS probe head, and, for variable temperature experiments, using a 2.5 mm double resonance ( 1 H/ 19 F-X) CP-MAS probe and a Bruker Cooling Unit (BCU-II). The 19 F MAS spectra were recorded using a Hahn echo sequence with an interpulse delay equal to one rotor period. The 90 • pulse lengths were set to 1.25 µs (for SnF 2 and BaSnF 4 ) and 1.5 µs (BaF 2 ) and the recycle delays were set to 900 s (for SnF 2 ) and 300 s (for BaF 2 and BaSnF 4 ) using the 1.3 mm CP-MAS probe head. For the variable-temperature experiments, using the 2.5 mm CP-MAS probe head, the 90 • pulse length was set to 2 µs and the recycle delay was set to 10 s. The temperature inside the rotor was estimated from the chemical shift and spin-lattice relaxation time (T 1 ) of 79 Br in KBr powder . 19 F spectra are referenced to CFCl 3 and were fitted using the Dmfit software .