Samples of [NH4]M(HCO2)3 where M = Mn 2+ and Zn 2+ were produced by slow diffusion reactions following literature methods. Mn 2+ samples were produced by layering Mn(ClO4)2•4H2O, 0.1 M, over a combination of formic acid, 0.4 M, and NH4CO2H, 0.8 M in methanol, 16 mL. Powder samples for QENS were produced by combining multiple batches. Zn 2+ samples were produced by the same layering method with Zn(ClO4)2•4H2O, 0.2 M, over a combination of formic acid, 0.8 M, and NH4CO2H, 2.4 M in dried methanol, 16 mL. QENS samples were produced by a scaled-up synthesis in 40 mL of dried methanol. These samples are all undeuterated. Colourless single crystals were produced for SCND of ~1 mm 3 for M = Mn 2+ and ~4 mm 3 for M = Zn 2+ . Powder diffraction patterns were collected on a Rigaku Miniflex benchtop diffractometer, with Le Bail refinements confirming the purity of the samples found in the ESI as Figures and. Single-crystal neutron diffraction measurements were done using the SXD diffractometer and QENS measurements using the OSIRIS spectrometer, both at the ISIS neutron source (see ESI for further experimental details). H NMR spectra were obtained on a Bruker Avance III HD spectrometer operating at a 2 H NMR frequency of 61.42 MHz. Samples were packed into 4 mm zirconia rotors. The 2 H shift scale was referenced with respect to neat TMS by setting the peak of a replacement sample of D2O to 4.81 ppm. Variable-temperature 2 H wideline (static) spectra were acquired using a solid echo with a 10 s recycle delay and 40 μs echo delay, starting at ambient temperature and cooling in stages down to -109 °C (calibrated temperature). Reproducibility of the spectra was observed on warming the sample to ambient temperature. The lineshapes corresponded to symmetric quadrupole coupling tensors (i.e. negligible asymmetry factor / rhombicity, η), and so the effective 2 H quadrupolar coupling, CQ, could be simply obtained from the separation of the peaks, Δν, using 𝐶 Q = 4Δ𝜈/3. Note that this analysis assumes a single dominant value of CQ.