CASSCF/NEVPT2 calculations performed on the ReBr6 2-guest in 3 suggests that the major contribution to DRe comes from the spin-flip transition |dyz/xz  dxy|. Due to the relatively weak π-donor nature of the Br ion, the splitting between the dyz/xz and dxy orbitals is found to be rather small, facilitating strong in-plane anisotropy (Tables and -12). In order to estimate the JCu-Cu and JCu-Re exchange interactions DFT calculations have been performed on complexes 1 and 3. Calculations suggest no interaction between the two Cu II ions in 1 and 3, but a non-negligible antiferromagnetic JCu-Re = -0.15 cm -1 in 3 leading to an S = ½ ground state (Table , see ESI for computational details). Spin density analysis shows the exchange is primarily mediated via the Re IV -Br•••Cu II pathway (Figure ). Overlap integral calculations between singly occupied molecular orbitals (SOMOs) of the paramagnetic ions, which helps in analysing the sign and magnitude of magnetic exchange interactions, reveal one moderately strong orbital interaction between the Re IV -Br•••Cu II centres (Cu(dx 2 -y 2 )|Br3|Re(dyz); Figure , S14, Table ) leading to an antiferromagnetic interaction. In the other two possible interactions (Cu(dx 2 - y 2