Magnetic property measurements were first carried out on a Quantum Design MPMS superconducting quantum interference device (SQUID; School of Chemistry, University of Nottingham, a). Additional isothermal magnetisation measurements were carried out on a Quantum Design Dynacool Physical Property Measurement system (PPMS; Cavendish Lab, University of Cambridge, b). Data were corrected for the diamagnetism of the sample using Pascal's constants. FeCl 2 (pym) a: A polycrystalline sample of FeCl 2 (pym) (4.5 mg) was immobilised in eicosane (5.9 mg) and sealed in a gelatin capsule. Magnetic susceptibility measurements were performed under field cooled (FC) and zero-field cooled (ZFC) conditions in a 0.01 T dc field from 2 K to 300 K. Isothermal magnetisation measurements were performed at 2 K from 0 T to 5 T to -5 T to 5 T.