This article reviews the most relevant chemical and structural aspects that influence the spin-crossover phenomenon (SCO). Special attention is focussed on the recent development of SCO coordination polymers. The different approaches currently being explored in order to achieve multifunctionality in SCO materials are discussed.

Thermal, pressure and light switchable spin-crossover materials

The article deals with coordination compounds of iron(II) that may exhibit thermally induced spin transition, known as spin crossover, depending on the nature of the coordinating ligand sphere. Spin transition in such compounds also occurs under pressure and irradiation with light. The spin states involved have different magnetic and optical properties suitable for their detection and characterization. Spin crossover compounds, though known for more than eight decades, have become most attractive in recent years and are extensively studied by chemists and physicists. The switching properties make such materials potential candidates for practical applications in thermal and pressure sensors as well as optical devices. The article begins with a brief description of the principle of molecular spin state switching using simple concepts of ligand field theory. Conditions to be fulfilled in order to observe spin crossover will be explained and general remarks regarding the chemical nature that is important for the occurrence of spin crossover will be made. A subsequent section describes the molecular consequences of spin crossover and the variety of physical techniques usually applied for their characterization. The effects of light irradiation (LIESST) and application of pressure are subjects of two separate sections. The major part of this account concentrates on selected spin crossover compounds of iron(II), with particular emphasis on the chemical and physical influences on the spin crossover behavior. The vast variety of compounds exhibiting this fascinating switching phenomenon encompasses mono-, oligoand polynuclear iron(II) complexes and cages, polymeric 1D, 2D and 3D systems, nanomaterials, and polyfunctional materials that combine spin crossover with another physical or chemical property.

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Spin state switching in iron coordination compounds

We analyze a series of recent experiments in which it has been found that pressure induces changes in charge or spin of iron [i.e., Fe(III) reduces to Fe(II) and in Fe(II) there are spin changes involving either decrease or increase of multiplicity]. In addition to considering the volume difference of the two states at zero pressure, we find it important to include effects both of the difference in compressibility of the two states, and the change in compressibility of a given state with pressure. The theory casts the problem in terms of the changes in Coulomb energy, closed shell repulsions, and both covalent bonding energy and crystal field energy accompanying the change in electronic state. In addition, interactions between converted iron atoms are included by a form of mean field theory and the effects are shown to be significant. Not only is the theory discussed analytically, but also a simple graphical solution is shown which makes it possible to examine readily the qualitative effects of the various parameters. Repulsive interactions spread the conversion over a larger pressure range and may thus explain why so many compounds exhibit rather broad transitions. Attractive and repulsive interactions can lead to cooperative effects. They should account for the reversible yet discontinuous jumps in conversion as a function of temperature previously observed in several phenanthroline and dipyridyl compounds. The possibility of hysteresis is also indicated. Major anomalies in the combined temperature and pressure variation of the conversion are accounted for by including the temperature dependence of the free energy of interaction.

Pressure‐Induced Electronic Changes in Compounds of Iron

Spin-state transitions in metal complexes which are driven by a change of temperature T or pressure p are always associated with a considerable reorganization of molecular geometry. The change involves metal-ligand bond lengths R, bond angles, and a variation of ligand orientation. In particular, the elongation 4R by up to ∼ 10% occurring in the course of the LS → HS conversion produces an expansion of molecular volume ΔV ≌ 25 A3 per metal atom. The average crystal structure for given values of T and p is reproduced by the fractional occupancy of the individual structures of the high-spin (HS) and low-spin (LS) isomer. The transitions are reasonably well described by a number of theoretical models which are equivalent to the Bragg and Williams approximation of the Ising model. The dynamics of the spin-state transitions in solution, based on measurements by ultrasonic and photo-perturbation techniques, is in general rapid with rate constants between 4 × 105 and 3 × 108 s−1. Similar results are obtained for the spin conversion in solid complexes where the line shape analysis of Mossbauer spectra based on the theory of Blume and Tjon is applied. The dynamics may be rationalized employing one-dimensional cross sections through Gibbs free-energy surfaces G = G(R), an alternative being the comparison of the results with quantum-mechanical calculations for a radiationless non-adiabatic multiphonon process.

Nature and dynamics of the spin-state interconversion in metal complexes

Spin Transitions in six-coordinate iron(II) complexes

Transition metal complexes with configurations d$^4$, d$^5$, d$^6$ and d$^7$ are of two types-high-spin and low-spin-depending on the strength of the ligand field. Ligand field theory predicts that truly octahedral complexes ML$\_6$ could exist in which both types are in thermal equilibrium at ordinary temperatures, but no completely unambiguous examples of this 'crossover' situation have yet been put forward. The magnetic and spectral properties associated with nearly equi-energetic high- and low-spin states should be singularly sensitive to temperature, pressure, and minor chemical modifications of the ligating molecules. These matters are discussed in detail, for the configuration d$^5$, with particular emphasis on the variation of total molecular energy with change in metal-ligand separation, and on a necessary inequality, namely $\Delta$ (high-spin) < $\pi$ < $\Delta$ (low spin) ($\Delta$ = ligand field strength, $\pi$ = mean pairing energy). Experimental data for certain FeS$\_6$-type compounds, viz. iron (III) N-N-dialkyldithiocarbamates, are then reported, which reproduce qualitatively all the requirements so formulated. The reciprocal magnetic susceptibility passes through a maximum and then a minimum with increasing temperatures and also increases sharply with applied pressure; the electronic spectrum is temperature dependent; and different alkyl substituents in the ligand drastically affect the magnetism. The values of $\Delta$ and $\pi$, as obtained from independent evidence, conform with the inequalities previously mentioned. The temperature dependence of the magnetism does not exactly conform with the predictions of the conventional van Vleck equation, but is considered to be tractable when vibrational partition functions are introduced into this equation. The pressure dependence of the magnetism leads to an estimate of the difference in molar volume of the two states, from which it is concluded that the Fe-S distances differ by about 0.07 $\overset{\circ}{\mathrm A}$, that in the low-spin state being the shorter.

Anomalous Behaviour at the $^6$A$_1$-$^2$T$_2$ Crossover in Iron (III) Complexes

The magnetic anisotropy of binuclear copper(II) acetate monohydrate has been redetermined by a null deflexion method at temperatures between 80 and 300 K. The new data establish that all previously reported anisotropy measurements have been seriously in error, presumably due to difficulties in correctly identifying the principal crystal axes. The previous necessity to invoke temperature-dependent values for the exchange integral J$\_{\text{eff}}$ and the g-factors now disappears and the new experimental anisotropies conform closely to Bleaney & Bowers (1952 b) theory with constant values for the disposable parameters. The magnetic anisotropy is especially sensitive to anisotropy in the high-frequency term of the susceptibility expressions. This enables the controversial electronic spectrum to be assigned in terms of the ligand field transitions 11 000 cm$^{-1}$ (d$\_{z^{2}}$ $\leftarrow $ d$\_{x^{2}-y^{2}}$); 14 400 cm $^{-1}$ (d$\_{xz,yz}$ $\leftarrow $ d$\_{x^{2}-y^{2}}$); 17 000 cm $^{-1}$ (d$\_{xy}$ $\leftarrow $ d$_{x^{2}-y^{2}}$). The nature of the chemical bonding and exchange interaction is discussed.

R. L. Martin

Papers

Anomalous Behaviour at the $^6$A$_1$-$^2$T$_2$ Crossover in Iron (III) Complexes

The Magnetic Anisotropy and Electronic Structure of Binuclear Copper(II) Acetate Monohydrate