The DMI vector, V, which favours a pair of spins being perpendicular to both it and each other, is by symmetry for NiCl 2 (pym) normal to the pyrimidine ring. This symmetry is broken in the monoclinic structures, and instead is merely confined to the plane normal to the M-M vector. Nevertheless, as this symmetry breaking is not large, we can assume as a first approximation that the component of V within the ligand plane is small, and that V lies along the plane normal. Within this approximation, the canting angle in the ordered ground state will be γ = 2 arctan(V /J L ) In this model, the DMI would be approximately 30% the size of superexchange for Ni and roughly equal to J in size for Fe if the canting is driven by DMI alone. These would be large values for the DMI interaction compared to other known compounds. Both Ni(II) and Fe(II) are expected to have significant single-ion anisotropy. In both cases the local ligand field environment can be thought of as 'compressed' as the four weakerfield π-donor Cl -ions lie in the equatorial plane, and the σ-donor N-heterocycles are axial. For d 8 Ni(II) this leads to a strong easy-axis (Ising) type anisotropy, and for d 6 Fe(II) this tends to produce an easy-plane (XY) type anisotropy. As the true symmetry is below tetragonal, there will be additional small rhombic anisotropy, E, neglected in this approximate treatment. There are two key parameters: the angle between the easy-axis and the M-M vector, ϕ, and the strength of the single ion anisotropy D (Fig. ). ϕ = 0 and 90 • corresponds to collinear anisotropy and hence will produce a collinear ground state, and ϕ = 45 favours a maximally canted state, which unusually has four degenerate ordered ground states. For NiCl 2 (btd), assuming the easyaxis is coincident with the N-M-N axis gives ϕ = 22.0(5) • and NiCl 2 (pym) ϕ = 31.1(5) • , due to the larger angle between the coordinating nitrogens in pym and btd, and the canting angles are ϕ = 9.1(3)) • for NiCl 2 (btd) and ϕ = 6.4(3) • for NiCl 2 (pym) (Tab. 1). Considering a Hamiltonian only containing single-ion anisotropy and Heisenberg AFM interactions for the Ni-L-Ni chain, analogous to that used in Pianet et al. , gives D J L = sin(γ) sin(ϕ-γ)cos(ϕ-γ) . Using the experimental values this implies that D/J L = 1.4(1) for NiCl 2 (btd) and D/J L = 0.58(3) for NiCl 2 (pym), broadly consistent with D observed in similar materials. The DMI and single-ion anisotropy terms will act cooperatively, and so the determined parameters thus correspond to estimates of the maxima rather than the central values. Our DFT calculations and Curie-Weiss analysis suggest that J pym is significantly larger than J btd , which would reduce the observed canting, and so the observed opposite trend suggests that the noncollinear interactions (D and V ) are in fact larger for NiCl 2 (btd).