• 𝑠𝑠 ⃗ � 𝑖𝑖 to a model that assumes just two independent exchange interactions, Jcr and Jrr, describing the interaction between the central Ni ion and ring Ni ions, and between ring Ni ions, respectively. The fit can be improved by adding additional Jcr and Jrr interactions (J1-J4, Figure ), permitted given the presence of just inversion symmetry in 1 (i.e. half the cluster is in the symmetric unit). In all cases the Jcr interaction is found to be ferromagnetic and the Jrr interaction is found to be antiferromagnetic. However, this simple isotropic model does not fit the magnetization data in the full temperature and field range, particularly at high fields. Introduction of an anisotropic term 𝐻𝐻 � = 𝐷𝐷 ∑ �𝑠𝑠 ⃗ ̂𝑖𝑖� 2 𝑖𝑖 to account for the zero-field splitting associated with the octahedral Ni II ions is therefore required, one for the central Ni ion with an {NiO6} coordination sphere (D1) and one for the ring Ni ions with {NiO4N2} coordination spheres (D2). The best fit of the susceptibility and magnetisation data with this model affords J1 = +4.13 cm -1 , J2 = -4.20 cm -1 , J3 = +3.93 cm -1 , J4 = -0.80 cm -1 , D1 = +4.71 cm -1 and D2 = -3.02 cm -1 . Note the opposite signs of D1 and D2, positive for the central Ni II ion and negative for the ring Ni II ions.