The spin transition is an example of transition between two electronic states in molecular chemistry, which have nearly identical energy levels. The ability of an electron to transit from a stable to another stable (or metastable) electronic state in a reversible and detectable fashion, makes these molecular systems appealing in the field of molecular electronics.

In octahedral surroundings

When a transition metal, is in octahedral surroundings, its ground state energy gap metal orbitals relative to the mean (see Crystal field theory, Hund's rule is obeyed. The HS ground state has got the same multiplicity as the free metal ion are comparable, a LS↔HS transition may occur.

d^{n} configurations

are by far the most important. The spin transition phenomenon, in fact, was first observed in 1930 for tris (dithiocarbamato) iron(III) compounds. On the other hand, the iron(II) spin transition complexes were the most extensively studied: among these two of them may be considered as archetypes of spin transition systems, namely Fe(NCS)2(bipy)2 and Fe(NCS)2(phen)2 (bipy = 2,2'-bipyridine and phen = 1,10-phenanthroline).

Iron(II) complexes

We discuss the mechanism of the spin transition by focusing on the specific case of iron(II) complexes. At the molecular scale the spin transition corresponds to an interionic electron transfer. It follows that the average metal-ligand bond length is longer in the HS state than in the LS state. This difference is in the range 1.4–2.4 pm for iron(II) compounds.

To induce a spin transition

. Among these techniques, Mössbauer spectroscopy has been particularly useful in the case of iron compounds, showing two well resolved quadrupole doublets. One of these is associated with LS molecules, the other with HS molecules: the high-spin molar fraction then may be deduced from the relative intensities of the doublets.

Types of transition

Various types of transition have been observed. This may be abrupt, occurring within a few kelvins curves may be strictly identical in the cooling or heating modes, or exhibit a hysteresis: in this case the system could assume two different electronic states in a certain range of temperature. Finally the transition may occur in two steps.

See also

• Spin crossover

References