Spin crossover

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Diagram to illustrate the dependence of the high-spin (HS) or low-spin (LS) state on the ligand field splitting Δ O in the octahedral ligand field and the associated electron configuration.

The transition between two (meta) -stable states is referred to as spin crossover , whereby one of the states has a lower multiplicity (low spin, few unpaired electrons) and the other a higher multiplicity (high spin, many unpaired electrons).

For elements with the electron configuration d 4 - d 7, there are two options for distributing electrons to the orbitals. Either one first fills all d-orbitals according to Hund's rule with one electron each of the same spin and then distributes the remaining ones with opposite spin to the energetically lowest orbitals. Or one initially only fills the area through the energetic splitting ( ligand field theory) lower d orbitals with paired electrons and then distributes the remaining ones to the energetically higher d orbitals. The state with the greatest possible number of unpaired electrons is called high spin, the state with the minimum number of unpaired electrons is called low spin. The change from low-spin to high-spin can be achieved with a suitable size of the ligand field splitting Δ by adding energy (e.g. heat or pressure). This change in the electronic molecular structure changes various physical properties of the material in question. For example, the magnetic behavior, the structure and the optical behavior (color, refractive index) of the material change.


Spin crossover was first observed by L. Cambi and L. Szegö in 1931 when they were studying the anomalous magnetic properties of tris (N, N-dialkyldithiocarbamato) iron (III) complexes under various conditions. The work on this phenomenon was followed up by Linus Pauling and co-workers with magnetic investigations on various heme derivatives of iron (II) and iron (III) complexes. They noticed that the spin state of these complexes is sensitive to the nature of their axial ligands. Orgel later proposed a possible balance of spin states as an explanation for the anomalous magnetic behavior within the framework of crystal field theory.

The first cobalt (II) spin crossover complex was reported by Busch and co-workers in the 1960s . As a result, the pioneering investigations by König and Madeja came in 1967, when they carried out extensive magnetic and Mössbauer spectroscopic investigations on iron (II) complexes and correctly interpreted the type of spin transition for the first time.

The rapid increase in interest in the spin crossover phenomenon has led to a better understanding of metal complexes and ligand field theory since then. Furthermore, spin crossover complexes are very promising materials that can be used in the future for the manufacture of switches, data storage devices, sensors or optical displays on a molecular basis.

Verification procedure

The most important consequences of the spin crossover are the changes in the metal-ligand bond lengths due to the occupation of the e g orbitals, which have a slightly antibonding character ( molecular orbital theory ) and the changes in the magnetic properties of the complex.

X-ray crystallography is used to measure the bond distances between the metal and the ligands, thus providing insight into the spin state of the complex.

The most important technique for characterizing spin transitions, however, is the measurement of the magnetic susceptibility as a function of temperature or the observation of the changing optical properties as a function of temperature ( SQUID ).

However, other analytical methods such as Mössbauer spectroscopy , NMR , IR , UV / VIS and Raman spectroscopy can also be used.

Individual evidence

  1. L. Cambi and L. Szegö: About the magnetic susceptibility of complex compounds . In: Chem. Ber. German Ges. . 64, No. 10, 1931, pp. 2591-2598. doi : 10.1002 / cber.19310641002 .
  2. CD Coryell, F. Stitt and L. Pauling: The Magnetic Properties and Structure of Ferrihemoglobin (Methemoglobin) and Some of its Compounds . In: J. Am. Chem. Soc. . 59, No. 4, 1937, pp. 633-642. doi : 10.1021 / ja01283a012 .
  3. RC Stoufer, DH Busch and WB Hadley: Unusual magnetic properties of some six-coordinate cobalt (II) complexes' electronic isomers . In: J. Am. Chem. Soc. . 83, No. 17, 1961, pp. 3732-3734. doi : 10.1021 / ja01478a051 .
  4. E. König and K. Madeja: 5T2-1A1 Equilibriums in some iron (II) -bis (1,10-phenanthroline) complexes . In: Inorg. Chem. . 6, No. 1, 1967, pp. 48-55. doi : 10.1021 / ic50047a011 .


  • P. Gütlich, HA Goodwin: Spin Crossover in Transition Metal Compounds I. Springer Berlin, 2004 , ISBN 978-3-540-40396-8