Having grown diffraction-quality crystals of FeCl 2 (pym), we determined its high temperature orthorhombic structure by SCXD at T = 120 K and the phase purity of the bulk microcrystalline sample was confirmed by PXRD at ambient temperature (Fig. , S2, Tab. S2, S3). The low temperature monoclinic structure of FeCl 2 (pym) was determined by Rietveld refinement of PND data at T = 12.5 K (Fig. , Tab. S4). In the absence of crystals suitable for SCXD measurements, the structure of NiCl 2 (pym) was determined by Rietveld refinement against PXRD data, using the structure of FeCl 2 (pym) as a starting model, as determined by SCXD (Fig. ). The monoclinic structures of FeCl 2 (btd) and NiCl 2 (btd) were determined by Rietveld refinement against PND data using DFT-optimised structures as a starting models (Fig. &). Simultaneous refinement of the nuclear and magnetic structure was undertaken for FeCl 2 (pym) and FeCl 2 (btd-d 4 ) against data collected at T = 2 K. However, the same analysis was not performed for NiCl 2 (pym) and NiCl 2 (btd-d 4 ) as the signal-to-noise ratio of the magnetic Bragg peaks was not sufficient to constrain the model. We first describe the general features of the structures, before going on to describe the crystal structure and refinements in detail.