WIELOTOROWE I NIEKONWENCJONALNE PRZEJŚCIA SPINOWE W ZWIĄZKACH KOORDYNACYJNYCH Fe(II)

Coordination compounds of 3d-block metal ions with electronic configurations 3d4-3d7 can exhibit the ability to change their spin state upon an applied external perturbation. This is accompanied by changes in magnetic, optical, and structural properties. Particularly significant changes are observed in Fe(II) coordination compounds, as the transition from the high-spin HS (t2g4eg2) to the low-spin LS (t2g6) form is accompanied by a loss of paramagnetic properties and a significant shortening of the Fe-donor bond length by approximately 10%. This change becomes an origin of perturbation affecting both the near and distant regions in the crystal lattice. As a result, this can lead to the occurrence of cooperative phenomena. The spreading perturbation can influence the structure of the building blocks. These changes may affect the conformation of flexible ligand molecules and the orientation of the donor group relative to the coordination octahedron. They can also affect larger structural elements, such as polymeric units, involving changes in their shape and mutual orientation. The coupling of electronic structure changes with structural changes can ultimately lead to a perturbation of the HS(T) spin transition curve (HS = molar fraction of HS form) and even the occurrence of complex behaviours - a compound with unchanged chemical composition can undergo a multiway spin transition. In such cases, structural variation occurring at the same perturbation factor (e.g., temperature) acts as a switch, allowing the selection of the path along which the spin state transition will occur. Major structural changes can even disrupt the conventional nature of the spin transition, leading to a combination of "normal" and "inverted" spin transitions. This paper reviews Fe(II) coordination compounds based on 1-substituted 1,2,3-triazoles and 2-substituted tetrazoles, which exhibit multipath and unconventional spin transitions. Keywords: