Intermolecular antiferromagnetism is the magnetic interaction in which unpaired electrons present on metal ions belonging to different molecules interact with one another and tend to align their spins in opposite directions. As a result, the magnetic moments of neighbouring molecules partially or completely cancel. Such compounds generally show lower magnetic susceptibility than expected for isolated paramagnetic ions and may approach a diamagnetic state at sufficiently low temperature. The interaction is an exchange interaction and is usually described by the Heisenberg exchange equation, (H=-2J S_1S_2). For antiferromagnetic coupling, (J<0), so the arrangement with antiparallel spins is energetically favoured. A suitable illustration is provided by neighbouring molecules containing Cu(II) ions, each having a (d^9) configuration and one unpaired electron. If two such molecules approach each other closely in the crystal, the singly occupied orbitals on the Cu(II) centres can interact. The interaction between metal centres of different molecules produces intermolecular antiferromagnetic coupling: [ \mathrm{Cu^{II}!-!L\cdots L!-!Cu^{II}} ] [ \mathrm{\uparrow \quad \downarrow} ] Here, the arrows represent the spins of the unpaired electrons on two Cu(II) centres in neighbouring molecules. The antiparallel arrangement has lower