Whereas there are hundreds of known iron(II) spin-crossover compounds, only a handful of cobalt(II) spin-crossover compounds have been discovered to date, and hardly an in depth study on any of them exists. This review begins with an introduction into the theoretical aspects to be considered when discussing spin-crossover compounds in general and cobalt(II) systems in particular. It is followed by case studies on [Co(bpy)3]2+ and [Co(terpy)2]2+ (bpy = 2,2'-bipyridine, terpy = 2,2':6',2″-terpyridine) presenting and discussing results from magnetic susceptibility measurements, X-ray crystallography, optical spectroscopy, and EPR spectroscopy

The spin transition of the [Co(terpy) 2 ] 2+ complex (terpy = 2,2 0 :6 0 ,2 00 -terpyridine) is analysed based on experimental data from optical spectroscopy and magnetic susceptibility measurements. The single crystal absorption spectrum of [Co(terpy) 2 ](ClO 4 ) 2 shows an asymmetric absorption band at 14 400 cm À1 with an intensity typical for a spin-allowed d-d transition and a temperature behaviour typical for a thermal spin transition. The single crystal absorption spectra of suggest that in this compound, the complex is essentially in the highspin state at all temperatures. However, the increase in intensity observed in the region of the low-spin MLCT transition with increasing temperature implies an unusual partial thermal population of the low-spin state of up to about 10% at room temperature. Finally, highspin ! low-spin relaxation curves following pulsed laser excitation for [Co(terpy) 2 ](ClO 4 ) 2 dispersed in KBr discs, and as a comparison for the closely related [Co(4-terpyridone) 2 ](ClO 4 ) 2 spin-crossover compound are given.

The manganese(iii) ion is ubiquitous among chemical systems that exhibit intriguing biological and physical properties, ranging from photosystem II and superoxide dismutase to the celebrated single-molecule magnet Mn 12 . It is for this reason that complexes and clusters of the manganese(iii) ion are widely recognized as the delicicae of high-field electron paramagnetic resonance (EPR) spectroscopists, [1] the fact that they invariably yield good-quality spectra being but a secondary consideration. [2][3][4][5][6][7] Hitherto, the spatial property of the hyperfine interaction between the S = 2 electronic spin and the I = 5/2 nuclear spin in any chemically relevant manganese(iii) compound has escaped observation. There are two fundamental reasons why knowledge of the hyperfine interaction energies is important. First, the magnitude and sign of the anisotropy relate directly to the manganese(iii) coordination sphere. Secondly, the hyperfine interaction has been demonstrated to be instrumental in quantum tunneling of the magnetization in singlemolecule magnets. [8,9] In an EPR study of manganese(iii) superoxide dismutase, [2] the hyperfine splittings were resolved and compared to those documented for a photo-oxidation product of manganese(iii) in the high-affinity site of photosystem II. Consistency was found between the observed hyperfine splitting and the sign of the zero-field-splitting parameter in the two systems. However, these experiments were confined to X-band parallel-mode experiments, which can determine just one component of the hyperfine interaction matrix. Information concerning the spatial property of the hyperfine interaction matrix requires the use of higher frequencies, as shown in a study of manganese(iii)-doped rutile. [12] In this communication we report single-crystal and powder EPR spectra of the [Mn(H 2 O) 6 ] 3+ ion at high fields and multiple frequencies. To minimize line broadening due to spin-spin dipolar interactions, the [Mn(H 2 O) 6 ] 3+ ion was doped into the diamagnetic cesium gallium alum, Cs-[Ga(H 2 O) 6 ](SO 4 ) 2 ·6 H 2 O, thus enabling the precise determination of the metal hyperfine interaction parameters.Pale orange-red crystals of this sample were grown at 0 8C from a saturated 6 m sulfuric acid solution of the cesium gallium alum that contained 1 % cesium manganese alum. A trace of the corresponding chromium(iii) alum was also added. The EPR lines of [Cr(OH 2 ) 6 ] 3+ were readily identified and were useful for crystal alignment.The single-crystal EPR spectra exhibited several magnetically nonequivalent species, reflecting the space group of the host and the propensity of the [Mn(OH 2 ) 6 ] 3+ ion to distort along a Jahn-Teller active coordinate. There are four tervalent cations in the cubic unit cell, each occupying a site of S 6 symmetry on one of the four unique threefold axes of the unit cell. In addition, at low temperatures the [Mn(OH 2 ) 6 ] 3+ ion is expected to be locked into one of the three possible tetragonally elongated structures that results from the JahnTelle...

a b s t r a c tThe spin-crossover compound [Fe(bbtr) 3 ](ClO 4 ) 2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane) forms a polymeric hexagonal sheet structure. It shows an abrupt thermal spin transition with 13 K wide hysteresis around 105 K, as evidenced by single crystal optical spectroscopy. The transition temperature for the thermal high-spin?low-spin transition on cooling as well as the relaxation kinetics just below T c ; depend upon the history of the sample. This is typical for a nucleation and growth mechanism and domain formation. In contrast, the high-spin?low-spin relaxation following the light-induced population of the high-spin state at low temperatures is governed by the intersystem crossing process.