Spin transition

The spin transition is one of the most spectacular example of transitionbetween two electronic states in molecular chemistry. The ability of to evolve from a stable to another stable (or metastable) electronicstate in a reversable and detectable fashion, makes these molecular systems appealing in the field of molecular electronics.When a transition metal ion of configuration $d^\{n\}$, $n=4$ to $7$,is in octahedral surroundings, its ground state may be low spin (LS)or high spin (HS), depending to a first approximation on the magnitudeof the $Delta$ energy gap between $e\_\{g\}$ and $t\_\{2g\}$ metal orbitalsrelative to the mean spin pairing energy $P$ (see Crystal field theory). More precisely, for$Delta>>P$, the ground state arises from the configuration wherethe $d$ electrons occupy first the $t\_\{2g\}$ orbitals of lower energy,and if there are more than six electrons, the $e\_\{g\}$ orbitals ofhigher energy. The ground state is then LS. On the other hand, for, Hund's rule is obeyed. The HS ground state has gotthe same multiplicity as the free metal ion. If the values of $P$and $Delta$ are comparable, a LS↔HS transition may occur.

Between all the possible $d^\{n\}$ configurations of the metal ion,$d^\{5\}$ and $d^\{6\}$ are by far the most important. The spin transitionphenomenon, in fact, was first observed in 1930 for tris(dithiocarbamato)iron(III) compounds. On the other hand, the iron(II) spin transitioncomplexes were the most extensively studied: among these twoof 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'-bypiridine and phen = 1,10-phenanthroline).

We discuss the mechanism of the spin transition by focusing on thespecific case of iron(II) complexes. At the molecular scale the spin transition corresponds to an interionicelectron transfer with spin flip of the transferred electrons. Foran iron(II)compound this transfer involves two electrons and the spinvariations is $Delta\; S=2$. The occupancy of the $e\_\{g\}$ orbitalsis higher in the HS state than in the LS state and these orbitalsare more antibonding than the $t\_\{2g\}$. It follows that the averagemetal-ligand bond length is longer in the HS state than in the LSstate. This difference is in the range 0.14-0.24 Åfor iron(II) compounds.

The most common way to induce a spin transition is to change the temperature of the system: the transition will be then characterized by a $ho\_\{H\}=f(T)$, where $ho\_\{H\}$is the molar fraction of molecules in high-spin state. Several techniquesare currently used to obtain such curves. The simplest method consistsof measuring the temperature dependence of molar susceptibility. Anyother technique that provides different responses according to whetherthe state is LS or HS may also be used to determine $ho\_\{H\}$. Amongthese techinques, Mössbauer spectroscopy has been particularly usefulin the case of iron compounds, showing two well resolved quadrupoledoublets. One of these is associated with LS molecules, the otherwith HS molecules: the high-spin molar fraction then may be deducedfrom the relative intensities of the doublets.

Various types of transition have been observed. This may be abrupt,occuring within a few Kelvin range, or smooth, occurring within alarge temperature range. It could also be incomplete both at low temperatureand at high temperature, even if the latter is more often observed.Moreover, the $ho\_\{H\}=f(T)$ curves may be strictly identical inthe cooling or heating modes, or exhibit a hysteresis: in this casethe system could assume two different electronic states in a certainrange of temperature. Finally the transition may occur in two steps.