We found that NiCl 2 (pym) and NiCl 2 (btd) both had ferromagnetic J Cl and antiferromagnetic J L consistent with our experimental ground states, bearing in mind the nonrelativistic nature of these calculations. Surprisingly, considering the experimental ground state, we found that FeCl 2 (pym) consistently had antiferromagnetic exchange in both directions. This provides further evidence that the level of theory we are using to probe the electronic states of these Fe(II) compounds is not sufficient to accurately characterise the physics of these systems, perhaps due to the unquenched orbital moment in octahedral high spin Fe(II) and the relevance of both t 2g and e g orbitals (which will have different localisation but are treated with a single U in our calculations). Further details on our calculations on the iron compounds can be found in the SI. In all cases the interlayer interactions were zero within error. Focussing now on the Ni(II) compounds, we find that the interactions through pym are much more antiferromagnetic than through btd. This is consistent with the experimental Curie-Weiss temperatures, which are much less positive for NiCl 2 (pym) (θ CW = +9(4) K) than for NiCl 2 (btd) (θ CW = +22(2) K), though the presence of non-Heisenberg interactions prevents detailed quantitative comparison. Examination of the spin-density reveals that for both Ni(II) compounds e g -type d-orbitals predominate, as predicted, and the ligand spin-density primarily lies within the σ-type orbitals, for Cl -, pym and btd (Fig. ).