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.

a b s t r a c tThe thermal and the light-induced spin transition in [Fe(bbtr) 3 ](ClO 4 ) 2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)) as well as the high-spin ? low-spin relaxation following the light-induced population of the high-spin state below the thermal transition temperature are discussed in relation to the accompanying crystallographic phase transition. The experimental data have exclusively been obtained using optical single crystal absorption spectroscopy.

We report a detailed DFT study of the energetic and structural properties of the spin-crossover Co(ii) complex [Co(tpy)(2)](2+) (tpy = 2,2':6',2''-terpyridine) in the low-spin (LS) and the high-spin (HS) states, using several generalized gradient approximation and hybrid functionals. In either spin-state, the results obtained with the functionals are consistent with one another and in good agreement with available experimental data. Although the different functionals correctly predict the LS state as the electronic ground state of [Co(tpy)(2)](2+), they give estimates of the HS-LS zero-point energy difference which strongly depend on the functional used. This dependency on the functional was also reported for the DFT estimates of the zero-point energy difference in the HS complex [Co(bpy)(3)](2+) (bpy = 2,2'-bipyridine) [A. Vargas, A. Hauser and L. M. Lawson Daku, J. Chem. Theory Comput., 2009, 5, 97]. The comparison of the and estimates showed that all functionals correctly predict an increase of the zero-point energy difference upon the bpy → tpy ligand substitution, which furthermore weakly depends on the functionals, amounting to . From these results and basic thermodynamic considerations, we establish that, despite their limitations, current DFT methods can be applied to the accurate determination of the spin-state energetics of complexes of a transition metal ion, or of these complexes in different environments, provided that the spin-state energetics is accurately known in one case. Thus, making use of the availability of a highly accurate ab initio estimate of the HS-LS energy difference in the complex [Co(NCH)(6)](2+) [L. M. Lawson Daku, F. Aquilante, T. W. Robinson and A. Hauser, J. Chem. Theory Comput., 2012, 8, 4216], we obtain for [Co(tpy)(2)](2+) and [Co(bpy)(3)](2+) best estimates of and , in good agreement with the known magnetic behaviour of the two complexes.

The electronic structures of mononuclear and dinuclear iron(iv) complexes are studied using magnetic circular dichroism and wavefunction-based ab initio methods, and then correlated with their similar reactivities toward H- and O-atom transfer.

F64Pc2Ln (1Ln, Ln = Tb or Lu) represent the first halogenated phthalocyanine double-decker lanthanide complexes, and 1Tb exhibits single-molecule magnet properties as revealed by solid-state magnetometry. The fluorine substituents of the phthalocyanine rings have a dramatic effect on the redox properties of the F64Pc2Ln complexes, namely, a stabilization of their reduced states. Electrochemical and spectroelectrochemical measurements demonstrate that the 1Tb(-/2-) and 1Tb(2-/3-) couples exhibit redox reversibility and that the 1Tb(-), 1Tb(2-) and 1Tb(3-) species may be prepared by bulk electrolysis in acetone. Low-temperature MCD studies reveal for the first time magnetization hystereses for the super-reduced dianionic and trianionic states of Pc2Ln.