The structure of FeCl 2 (pym) determined from SCXD data collected at T = 120 K and Rietveld refinement against PXRD data collected at ambient temperature, show it crystallises in the orthorhombic space group P mmb (Tab. S2, Fig. ). However, refinement of the nuclear structure against PND data collected at T = 1.5, 12.5 and 25 K reveals that FeCl 2 (pym) is in the monoclinic P 2 1 /m space group at these temperatures (Fig. , Tab. S4). This structural phase transition can be seen in peak splitting of the peak at Q = 3.66 Å and was confirmed by Rietveld refinement, where the monoclinic structure refines β = 90.920(9) • with a significantly improved fit (R wp = 2.930, vs R wp = 3.854 for β = 90 • ). We allowed the orientation of the pym to refine while keeping it as a rigid body. Our single crystal structure of FeCl 2 (pym) was then used as a starting model for Rietveld refinement of the structure of NiCl 2 (pym), initially against laboratory PXRD data (Fig. ), and then against PND data at T = 25 K. We found no evidence of a structural transition in this material down to 2 K.