Chromium, tris isomerism

Chromium, tetraaquadichloro-chloride dihydrate hydrate isomerism, 1, 183 Chromium, tetrabromo-solvated, 3, 758 synthesis, 3, 763 Chromium, tetrachloro-antiferromagnetic, 3, 761 ferromagnetic magnetic properties, 3,7559 optical properties, 3,759 structure, 3,759 solvated, 3. 758 synthesis. 3, 759 Chromium, tetrachlorooxy-tetraphenylarsenate stereochemistry, 1,44 Chromium, tetrahalo-, 3,889 Chromium, tetrakis(dioxygen)-stereochemistry, 1,94 Chromium, triamminediperoxy-structure. 1, 78 Chromium, tricyanodiperoxy-structure, 1, 78 Chromium, trifluoro-electronic spectra, 3, 757 magnetic properties, 3, 757 structures, 3, 757 synthesis, 3, 756 Chromium, trihalo-clcctronic spectra, 3, 764 magnetic properties, 3, 764 structure, 3, 764 synthesis, 3, 764 Chromium, tris(acetylacetone)-structure. 1, 65 Chromium, tris(bipyridyl)-... 

Chromium, tris(hexafluoroacetylacetonato) optical isomerism, 1, 28 Chromium, tris(oxalato)-raceinization solid state, 1,466 strychnine salt... 

Chromium, tris(hexafluoroacetylacetonato)-optical isomerism, 28 Chromium, tris(l,10-phenaiithroline)-ligand field photochemistry, 398 Chromium, tris(trifluoroacetylacetonato)-isomerism, 28 Chromium complexes geometric isomerism, 11 Chromium (0) complexes magnetic behavior, 273 Chromium(I) complexes magnetic behavior, 272 Chiomium(II) complexes magnetic behavior, 272 spectra, 252 thiocyanate... 

Chromium(II) complexes of bipyridyls, terpyridyl and the phenanthrolines have been discussed in Section 35.2.2.1. Complexes of the ligands 2-aminomethylpyridine (pic, 2-picolyl-amine) and 8-aminoquinoline (amq), which have one heterocyclic and one amino nitrogen donor atom, have been prepared by methods similar to those in Scheme 10. The bis(amine) complexes are typical high-spin, distorted octahedral complexes, and the mono(amine) complexes, from their antiferromagnetic behaviour and reflectance spectra, are six-coordinate, halide-bridged polymers (Table 15).103 No tris(amine) complexes could be prepared so the attempt to find spin isomeric systems in octahedral chromium(II) systems was unsuccessful ([Cr(en)3]X2 are high-spin and [Cr(bipy)3]X3 and [CrX2(bipy)2] low-spin). 

A rate and equilibrium study of the cis/trans isomerization of tris(l,l,l-trifluoro-2,4-pentanedionato)chromium(III) in the vapour phase has been reported.761 The equilibrium constant 3.56 was independent of temperature in the range 118-144.8 °C, the same value being measured by both static and kinetic methods. The activation energies for isomerization were both less than 115 kJmol-1 the Cr—O bond energy is 210 kJ mol-1. On these grounds, a bond-rupture mechanism was rejected. A similar mechanism may operate for the isomerization in solution. 

Complexes of salicylate with chromium(III) have not been reported but the tris complexes of salicylaldehyde and chromium(III) may be prepared by refluxing [Cr(THF)3Cl3] with salicylal-dehyde and sodium acetate in ethanol.939 The acid hydrolysis of this complex was studied in detail, but the isomerism obviously possible for this complex was not apparently considered. Khan and Tyagi940 studied the formation of phthalate complexes of chromium(III). 

Complexes of simple amino adds with chromium(lll) were first prepared by Ley.1148 The isomers possible for tris chelated complexes of this type are illustrated below (248-251). The consequences of such isomerism were first seriously considered by Gillard.1149 Red complexes of the formulae [Cr(gly)3] and [Cr(L-ala)3] were prepared by neutralizing refluxed solutions of hexaaquachromium(Ill) and the amino add in ratios between 1 5 and 1 10. These complexes were shown to be isomorphous with 0-[Co(gly)3] and D- -[Co(L-ala)3] respectively. The crystal structure of red j8-[Cr(gly)3] has also been reported.1150... 

It must be concluded, therefore, that the ligands do not become completely detached from the metal ion in isomerization reactions. Comparable results have been observed in the isomerization95 of potassium diaquodioxalatochromium(III) and the racemization96 of optically active potassium tris(oxalato)chromium(III) when no exchange with free ligand in solution occurs. Thus, although it is not practicable to take advantage of the desirable properties of individual isomers of 2 1 chromium and cobalt complexes of tridentate azo compounds because of the facility with which such compounds isomerize in solution, the technically important unsymmetrical 2 1 complexes are capable of practical application because they show little or no tendency to disproportionate in solution. 

N-methyl derivative resulted in oxidation of the ligand with concomitant reduction of Co (III) to Co (II). The preparation of tris (benzohydroxa-mato) chromium (III), Cr(benz)3, was successful and resulted in the separation and characterization of its two geometric isomers (2). The half-lives for isomerization of these complexes near physiological conditions is on the order of hours. To facilitate the separation of all four optical isomers of a simple model tris (hydroxamate) chromium (III) complex, we prepared (using Z-menthol as a substituent) the optically active hydroxamic acid, N-methyl-Z-menthoxyacethydroxamic acid (men). This resulted in the separation of the two cis diastereoisomers of tris(N-methyl-Z-menthoxyacethydroxamato) chromium (III) from the trans diastereoisomers and their characterization by electronic absorption and circular dichroism spectra. 

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