Chromium complexes magnetic properties. 701

Chromium oxide has a rather complicated electronic structure. It is antiferromagnetic. The so-called spinwaves are difficult to take into account in periodic calculations. The band gap, approximately 3.4 eV, is a bit larger than the one of Ti02. The very complex magnetic properties make the choice of the functional for DFT calculations very delicate. 

The electronic301 and magnetic properties of mononuclear chromium(III) complexes are quite well understood however there is a distinct tendency for octahedral symmetry to be invoked in cases where the true symmetry is much lower. Chromium(III) is a hard Lewis acid and many stable complexes are formed with oxygen donors. In particular hydroxide complexes are readily formed in aqueous solution, and this may be a problem in synthesis. Substitution at chromium(III) centres is slow302,303 and may well have some associative character in many cases. The kinetic inertness of chromium(III) has led to the resolution of many optically active complexes this work has been extensively reviewed.304... 

The magnetic method cannot be applied to tripositive chromium, the structures of the two extreme types having the same number of unpaired electrons and entering into resonance with each other. The chemical properties of the chromium complexes indicate that chromium, like the other iron-group elements, forms hyperligating bonds... 

Only a very few polynuclear complexes containing more than two chromium(III) centers have been studied so far. However, magnetochemical and inelastic neutron scattering studies, heat capacity measurements, and emission spectroscopy have been reported for various tetranuclear species (40,142 151). Two review articles dealing with the spectroscopic and magnetic properties of chromium(III) oligomers have recently appeared (127, 128). 

The chromium(II) cation [Cr(terpy)2] is conveniently prepared by the interaction of aqueous chromium(II) solutions with excess terpy, and may be isolated as its perchlorate or iodide salt 239,255,299). The magnetic properties of [Cr(terpy)2][C104]2 have been recorded over the temperature range 20-300 K the complex is low spin its exhibits a nearly linear temperature dependence over this range 299). The complex may be reduced electrochemically (255) or chemically 239). Herzog and Aul have isolated the various [Cr(terpy)2]" complexes that may be obtained by chemical reduction [Cr(terpy)2]l2 (red-brown, = 2.80 BM), [Cr(terpy)2]I (wine-red, = 1.85 BM), and [Cr(terpy)2] (green, /r ff = 0.63 BM) 239). 

The chromium sulfide system is very complex, with two forms of Cr2S3 and several intermediate phases between these and CrS. Rhombohedral Cr2S3 has complex electrical and magnetic properties. The mixed-valent telluride Cr3Te4 is ferromagnetic. 

The chromium(III) complexes represent the most simple systems in which magnetic interactions between paramagnetic ions can be studied. The cobalt(lll) systems may serve as matrixes in which the intermolecular interactions between the paramagnetic ions can be diminished. In these connections it is noteworthy that a mixed dihydroxo complex, (-)s89-A,A-[(en)2Cr(OH)2Co(en)2]has been described recently. This compound may serve as a model compound that represents the magnetic properties of the individual chromium(III) ion of the di-/a-hydroxochromium(lII) complex in the absence of a pair interaction. 

Chromium(lll) is the archetypal d ion and the electronic spectra and magnetic properties of its complexes have therefore been exhaustively studied (see Panel). Data for a representative sample of complexes are given in Table 23.6. 

Picolinic acid (pyridine 2-carboxylic acid) complexes of chromium(III) have been the subject of a number of studies. Complexation by picolinic acid in water/ethanol (30% v/v) follows an ion-pairing, Eigen-Wilkins type mechanism.Activation parameters suggest an associative character for the reaction of the aqua complex. Chelated complexes of chromiuni(ni) and picolinic acid are the products of the rapid, inner-sphere reduction of [Co (pico)(NH3)5p with chromium(II). The reaction of the related 4-carboxylic acid complex of cobalt(III) with chromium(II) is also rapid in contrast, pyridine-3-carboxylic acid (nicotinic acid) complexes undergo slower reactions. A -hydroxy-bridged dimeric complex [Cr2(pico)4(OH)2] has also been prepared. A study of magnetic properties in the temperature range 16-300 K leads to J - —6 cm and g = 2, typical for such complexes. 

Chromium(I) complexes, 702 alkyl isocyanides, 704-709 aryl isocyanides, 704-709 2,2 -bipyridyl, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 cyanides, 703 fluorophosphine, 716 isocyanides crystallography, 708 electrochemistry, 709 spectroscopy, 708 synthesis, 707 1,10-phenanthroline, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 tertiary phosphines dinitrogen, 713 tris(bipyridyl)... 

Table 5.6. EPR Parameters and Magnetic Properties of Carbene Complexes of Chromium(I) and Iron(I) ...

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