Spin-Crossover Complexes

Abstract

Spin-crossover (SCO) is a spin-state switching phenomenon between a high-spin (HS) and low-spin (LS) electronic configurations in a transition metal center. The SCO phenomenon is widely recognized as an example of molecular bistability. The SCO compounds most widely studied are six-coordinate first-row transition metal complexes with d 4 -d 7 configurations. A relative small enthalpy variation between LS and HS states can be realized by coopetition between ligand field stabilization (LFS) and spin pairing energies, which is illustrated by the Tanabe-Sugano diagrams in common coordination chemistry textbooks. Since an entropy variation in spin multiplicity from LS to HS is always positive, an increase in temperature can induce the transformation in Gibbs free energy from a positive to a negative sign, at which point SCO conversion occurs from the LS to HS state.Cambi and co-workers' pioneering work on the anomalous magnetic behaviors of mononuclear Fe(III) dithiocarbamate complexes [1] was first recognized as SCO phenomena in the early 1930s. However, progress on SCO complexes awaited the dissemination of ligand field theory into coordination chemistry. The concept of controlling LFS energies by substitution with different field strength ligands resulted in the corroboration of SCO phenomena in some Co(II) and Fe(II) complexes in the early 1960s. Moreover, subsequent findings that pressure [2] and light [3] can induce an SCO phenomenon may attract attention to SCO complexes. In the 1990s the demonstration of device applications using the SCO complex [4] illuminated the potential of SCO complexes in future practical applications in memory, display, and sensing devices. Figure 1 shows the number of published articles per year whose titles or topics contain the words "spin-crossover," "spin equilibrium," or their derivatives. Studies concerning SCO complexes have apparently increased since the 1980s; moreover, the number has more rapidly developed after the 2000s. The fundamentals and applications of SCO complexes have attracted growing interest not only in inorganic coordination chemistry but also in a wide range of relevant research fields.

Mononuclear High‐spin Octahedral Cobalt(II) Complex with Positive Axial Magnetic Anisotropy: Synthesis, Crystal Structure, and DFT Studies

“…The primary rise of magnetization shows a positive curvature up to 27 kOe. It becomes then almost linear up to 2.01 Nμ B in agreement with the fact that only the ground Kramers doublet is populated at 2 K , …”

“…It is well know that the cobalt(II) ion with 3d 7 electronic configuration in octahedral field may occur both in high spin (HS) and low spin (LS) state with maximum spin multiplicity (2 S + 1) = 4 and 2, in presence of weak and strong ligand field strength, respectively. Hence, there is a chance of spin crossover (SCO) between HS and LS states when the ligand field strength is very close to the crossover point being reversibly induced by magnetic field . Several studies exposed that SCO behavior of the cobalt(II) complexes; however, few of mononuclear HS cobalt(II) complexes (with S = 3/2 ground state) have been exposed to display SMM behavior , .…”

Monitoring the spin crossover phenomenon of [Fe(2‐mpz)2Ni(CN)4] 2D Hofmann‐type polymer nanoparticles via temperature‐dependent Raman spectroscopy

“…[ 27 ] The SCO Hofmann‐type complexes have been considered ideal candidates for chemosensing applications showing several advantages: magnetic properties affected by guest molecules, SCO temperatures near room temperature (RT), and retention of the SCO bistability in thin films and nanoparticles. [ 28 ] Hofmann‐type MOFs with the chemical structure [Fe(pyrazine)M(CN) 4 ] (M = Ni, Pt) are considered the most well studied among the Hofmann coordination polymers. [ 29–31 ] It is noteworthy that a careful choice of the bridging organic ligands and cyanometalate units could lead to preferable SCO characteristics of the material.…”

A Hidden Coordination-Bond Torsional Deformation as a Sign of Possible Spin Transition in Nickel(II)-Bis(nitroxide) Compounds

“…Magnetic switches are needed for applications in display, memory, sensor, and related smart devices. Iron(II) (3d 6 )-based spin-crossover (SCO) materials are the most intensively studied [ 9 , 10 , 11 , 12 , 13 , 14 ], because magnetic and chromic changes would be drastic between the low-spin (LS) S = 0 diamagnetic and high-spin (HS) S = 2 paramagnetic states. Furthermore, much attention has been paid to SCO near room temperature required for applications under ambient conditions.…”