where J ij is isotropic superexchange for nearest neighbour, J Cl , and next-nearest-neighbour, J btd interactions, and |S| = S(S + 1), and using a constant term to account for temperature dependent background scattering. The fit to these parameters show that J Cl = 1.6(4) K is strongest and ferromagnetic, whereas the superexchange through the ligand, J btd = -0.51(5) K is weaker and antiferromagnetic. Fitting with additional Heisenberg interaction terms, including J vdW , J 2Cl (i.e. the nextnearest neighbour superexchange along the FeCl 2 chain), did not improve the quality of the fit and the refined values of these additional values were an order of magnitude smaller and zero within error. Refinement of a model with Ising spin degrees of freedom (moment directions fixed to those determined from refinement of the ground state) produced a less physical scale factor. The limited data quality means our measurements are only weakly sensitive to the single ion anisotropy and other spinorbit derived interactions, and unfortunately prevented us from carrying out similar analysis for FeCl 2 (pym) and the Ni(II) containing compounds.