Lantern-like guest⸦[M2L4] 4+ coordination cages, normally associated with diamagnetic metals ions such as Pd II , can also be made with paramagnetic M II ions, here Cu II , through simple selfassembly of four molar equivalents of the ligand molecule (L = 1,3-bis(3-ethynylpyridyl)benzene) with two molar equivalents of the corresponding metal salt, followed by one molar equivalent of guest, forming [Cu II 2L4(H2O)(OTf)3](OTf)•MeCN (1), ReBr6⸦ [Pd II 2L4](BF4)2 (2) and ReBr6⸦[Cu II 2L4(OTf)2] (3), respectively. Complex formation was confirmed by ESI-MS and 1 H NMR, both revealing solution stability. Magnetic measurements combined with theoretical calculations show that: (a) there is no magnetic interaction between the Cu II ions in the "empty" [Cu2L4] 4+ cage, 1. (b) The geometry of the encapsulated [ReBr6] 2-ion remains essentially unchanged with respect to its metal salt and thus the axial zero-field splitting parameter, DRe, also remains the same. (c) The ingress of the ReBr6 2-ion induces a magnetic exchange interaction between the Re IV guest and the Cu II host, mediated primarily by the Cu(dx 2 -y 2 )||Re(dyz) orbitals in the Re-Br•••Cu pathway leading to dominant antiferromagnetic exchange and an S = ½ ground state for the complex. This was confirmed by overlap integral calculations and NBOs analysis where significant donor-to-acceptor interactions are observed between hybrid |4s 0.37 4px 0.63 | orbitals of the Br ions and the empty 4s, 4p orbitals of the Cu ions. The ability of paramagnetic host complexes to encapsulate paramagnetic guest complexes highlights some interesting possibilities for future work. These include: (1) the ability to switch on/off magnetic interactions through simple solution-based / redox chemistry, or through external perturbation, e.g. light, pressure, magnetic/electric fields. ( ) To specifically design host frameworks able to isolate/stabilise unstable/reactive magnetic molecules or those with unusual geometries/coordination numbers. The structural and physical characterisation of such species will have potential application across a breadth of electron-spin based quantum technologies. 4. References