Other Chromium III Complexes

Good evidence, including esr absorption, and circular dichroism spectra, has been reported for the formation of Cr(V)-nitrides from the uv photolysis of azidochromium(III) complexes such as [Cr(L)N3] (L = edta , nta , salen, or d-valine-iV,iV-diacetate ion).  

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Table 68 Visible Spectra of Isomeric Thiocyanato and other Chromium(III) Complexes ...

Table 8 summarizes data reported for other chromium(iii) complexes containing a variety of donor ligands. 

As has been seen, inert complexes of the transition metals may be inter-converted by substitution reactions, but such methods cannot generally be relied upon and it is preferable to prepare inert complexes by a different method. The chosen method is to take a compound containing the metal in a different oxidation state and oxidize or reduce it, as appropriate, in the presence of the selected coordinating ligand. This technique is used extensively in the preparation of oxalato complexes of chromium(III). Other chromium-(III) complexes are prepared by the oxidation of chromium(II) salts. 

On the other hand, if electron transfer does occur within this bridged complex, a bridged cobalt(ii) - chromium(iii) complex is generated (Eq. 9.36). The d cobalt(ii) center is labile whilst the d chromium(iii) center is inert. 

Other half-sandwich Cr complexes which show good activities for olefin polymerization include those with ether and thioether pendant arms (93) and (94) which show activities of 1,435gmmol-1 h-1 bar-1 and 2,010 gmmol-1 h 1 bar-1 respectively.252 The half-sandwich phosphine complex (95) affords a-olefins arising from chain transfer to aluminum,256,257 while the related boratabenzene chromium(III) complex (96) generates linear PF.258,259 Cationic species have also been investigated, and (97) polymerizes ethylene with an activity of 56 gmmol-1 h-1 bar-1.260-263... 

Seitz, Suydam, and Hercules 186> recently developed on the basis of luminol chemiluminescence a method for chromium-III ion determination which has a detection limit of about 0.025 ppb. The method is specific for free chromium-III ions as chromium-VI compounds have no catalytic effect and other metal ions can be converted to a non-catalytic form by complexing with EDTA, since the chromium-III complex of EDTA, which is in any case not catalytically active, is formed kinetically slowly 186>. To detect extremely small light emissions, and hence very small metal concentrations, a flow system was used which allows the reactants to be mixed directly in front of a multiplier. (For a detailed description of the apparatus, see 186>). 

It will become evident from this discussion of cobalt(III) and chromium(III) photochemistry that a fortunate combination of many factors makes these systems especially amenable to mechanistic investigation this is not the case for most other inorganic systems. For this reason, the progress made toward understanding the photochemical reactions of cobalt(III) and chromium(III) complexes may not be easily duplicated in other areas and it is useful to inquire whether conclusions reached here can be generalized. It is my belief that they can, and this and the next section are intended to place the subsequent discussion in the perspective of inorganic photochemistry. 

Both absorption and emission spectra have been recorded for a variety of octahedral chromium(III) complexes. For the systems of interest here, A/B 2. Inspection of Figure 2 leads to the expectation of three spin-allowed, parity-forbidden transitions between the iA2g and the other quartet states and two spin- and parity-forbidden transitions between the iA2g and the 2Eg and 2T2g states. Aqueous solutions of Cr(H20)s3+ display three bands with e 15 at 17,400, 24,500, and 38,000 cm-1, assigned respectively to the transitions iA2g- iT2g,... 

Other reported chromium(III) complexes containing py and related ligands are listed in Table 58. Complexes of 2-pyridylmethylamine are dealt with in Section 35.4.2.3 and of pyridine carboxylic acids in Section 35.4.8. 

The kinetics of aquation of a number of azidochromium(III) complexes have been investigated.303,655 Compared with other acidochromium(III) complexes, the chromium-azide bonds in these species seem remarkably stable to thermal substitution. Hence in the base hydrolysis of [CrN3(NH3)s]2+ a pathway involving initial loss of NH3 concurs with the usual base hydrolysis pathway involving loss of Nj. The aquation of azidochromium(III) complexes is H+-assisted with protonation of the azido ligand accounting for the enhanced reactivity. 

In comparison with some other metal ions, limited synthetic work has been carried out on chromium(III) complexes with macrocyclic ligands. The vast majority of reported complexes involve tetraazamacrocycles. 

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