Symmetry-breaking structural phase transitions in spin crossover complexes

We review spin-crossover (SCO) complexes of 3d transition metal ions that exhibit structural phase transitions (SPTs) associated with changes in the space group symmetry. An analysis of the existing cases extracted by data-mining the Cambridge Structural Database reveals that they can be classified according to the particular cause of symmetry breaking. One class of complexes exhibits the SPT caused by concerted displacive atomic motions due to the change in the metal-ligand bond lengths during the SCO. Such transitions are usually abrupt, with strong coupling between the SCO and SPT. The second class is characterized by symmetry lowering due to the stabilization of an intermediate mixed high-spin/low-spin state. The abruptness of the SCO is not a necessity in such case. Finally, in the third class, the SPT takes place due to the crystallographic ordering of peripheral groups, counter ions, or solvent molecules that are remote from the SCO centers. Such SPTs exhibit only weak coupling (if any) to the SCO. This analysis is expected to assist in the design of multifunctional SCO materials that rely on the symmetry-breaking SPTs. (C) 2014 Elsevier B.V. All rights reserved.