Geometry optimization of the primitive structures derived from single crystal X-ray diffraction data, FeCl 2 (pym) and CoCl 2 (btd) with Figure : Magnetic diffuse scattering of FeCl 2 (btd) fit using an effective field model. Data obtained by temperature subtraction of data measured at 30 K. the transition metal substituted as appropriate, produced structures consistent with those obtained by Rietveld refinement: with typical mismatches of less than 1%, and the largest deviations of 3% found for NiCl 2 (pym) (ESI Tab. S6). These calculations also found a small monoclinic distortion in FeCl 2 (pym), as found in our low temperature Rietveld refinement of neutron diffraction data. Examination of the electronic structure, including density of states and band structure, revealed that the inclusion of a U parameter was essential to avoid unphysically delocalised states. We explored a range of U values from U = 0 to 10 eV for both systems , determining that U = 2 eV for Fe and U = 6 eV for Ni were most appropriate. This correctly captured the experimentally observed insulating states for FeCl 2 (pym) (E g = 1.30 eV), NiCl 2 (pym) (E g = 2.48 eV) and NiCl 2 (btd) (E g = 1.52 eV), though we found that a metallic state results for FeCl 2 (btd), likely due to strong electronic correlations on Fe(II). We note that the chain like structure appears to afford significant delocalisation along the a direction, particularly for btd containing compounds.