Substitution reactions of Cr

It is generally believed (1) that the photochemical substitution reactions of Cr(C0)5 can be summarised ... 

Chromium produces some of the most interesting and varied chemistry of the transition elements. Chromium(O) and chromium(I) are stabilized in organometallics (Prob. 8). There have been extensive studies of the redox chemistry of Cr(II), Cr(III) and Cr(VI). Generally the Cr(IV) and Cr(V) oxidation states are unstable in solution (see below, however). These species play an important role in the mechanism of oxidation by Cr(VI) of inorganic and organic substrates and in certain oxidation reactions of Cr(II) and Cr(III). Examination of the substitution reactions of Cr(III) has provided important information on octahedral substitution (Chap. 4). 

The photochemical and thermal substitution reactions of Cr complexes can follow quite different stereochemical courses. The behavior described by equation (32) is typical thermal aquation of trans-[CrCl2(en)2] " proceeds with retention of configuration, whereas photoaquation is accompanied by a hi ly stereospecific ( 99% cis product) rearrangement. With very few exceptions, it has been found that stereochemical mobility is a general feature of Cr photosubstitution reactions. 

While little is known about the actual rates or equilibria of Cr(V) and Cr(IV) substitution reactions, Hintze and Rocek have been able to show from a study of the oxalic add catalysis of chromic acid oxidation of tris l,10-phenanthrohne)iron(II) that the substitution reactions of Cr(V) and Cr(IV) must be at least an order of magnitude greater than 4 X 10 NT s-. ... 

The cw-labilizing effect of oxoanions such as nitrate or acetate when co-ordinated to chromium(iii) was discussed in Volume 4 of this series. This is believed to occur through the formation of transient bidentate species through chelation of the oxoanion. From studies of the nitrate substitution reactions of [Cr(H20)50N02] + in aqueous DMSO, the rate constant at 298.2 K for water exchange with [Cr(HaO)5 ONOa] " is reported to be ca. 8 x 10 s , which is several orders of magnitude larger than for [Cr(H20)e] +, The rate constant for the aquation of [Cr(HaO)6-ONOa] + is 10 A = 8.8 s i at 298.2 K. 

A series of 1,3,2-diazaboroles in a thermal substitution reaction with [Cr(CO)3(AN)3] forms the j -coordinated complexes 59 (R = R = Me, Et, /-Pr R = Me, R = Et) (90IC4421). The corresponding dimeric ligand in this reaction yields complex 60 where only one heteroring is j -coordinated. 

Cr also catalyses some substitution reactions of the species Cr(NH3)5X, viz. 

Not every substitution of alkanesulfonyl chlorides, by any means, involves an elimination—addition mechanism. Thus, while the reactions of cr-toluene-sulfonyl chloride, PhCH2S02Cl, with either hydroxide ion, or primary and... 

The reactions of Cr(III), Al(III) and Gaflll) have been rationalized in a manner similar to that used for the reactions of Fe(lll). Although the hydroxy form MOH is the minor species present in the acid medium used in such studies (typically 1-2%), its enhanced reactivity compared with both in substitution and redox reactions, will ensure its... 

Of course the Co CNHj) breaks down rapidly in acid into Co + and 5NHJ. Precursor complex formation, intramolecular electron transfer, or successor complex dissociation may severally be rate limiting. The associated reaction profiles are shown in Fig. 5.1. A variety of rate laws can arise from different rate-determining steps. A second-order rate law is common, but the second-order rate constant is probably composite. For example, (Fig. 5.1 (b)) if the observed redox rate constant is less than the substitution rate constant, as it is for many reactions of Cr +, Eu +, Cu+, Fe + and other ions, and if little precursor complex is formed, then = k k2kz ). In addition, the breakdown of the successor complex would have to be rapid k > k 2). This situation may even give rise to negative (= A//° +... 

It was found that AV for both the acid-independent and acid-catalyzed pathways are approximately zero (109). Thus it was interpreted that Eq. (17) can be considered a substitution reaction of the Cr(III) central cation which proceeds through an interchange, I, mechanism... 

There is growing evidence that substitution reactions in Co(lll) complexes may not be typical of octahedral transition metal complexes. Early studies of substitution reactions for Cr(III) complexes revealed a rather strong dependence of reaction rate... 

Photocalorimetry offers a convenient alternative to other methods of AH determination and, in some instances, may be the only practical method. The ligand substitution reactions of robust Werner-type complexes are a case in point. Conventional thermochemical measurements are complicated by the slowness of the substitution process and/or by competing reactions. Some of these same complexes, however, undergo clean photosubstitutions with high quantum yields and thus are excellent candidates for photocalorimetry. Examples include [Cr(NH3)6]3+, [Cr(CN)6]3-and [Co(CN)6]3-.192 Photocalorimetric measurements of AH have also been obtained for isomerization and redox reactions of coordination compounds.193194... 

A comparison of the activation parameters Af/ ( j) for the ammonia and ethylenedi amine (meso isomer) complexes shows that Af/ (NH3) > Af/l(en) for both chromium(III) and rhodium(III), and that Af/ (Ir) > AHt(Rh) > AHt(Cr) for both NH3 and en. These trends are in keeping with the data for substitution reactions of the corresponding mononuclear complexes (364). 

The mono- and dinuclear paramagnetic complexes are of course extremely reactive. We have shown that the reactions of Cr(CO)J and Cr2(CO)10I with various mono- andbidentate ligands lead to the formation of the respective substitution products of Cr(CO)6 with simultaneous oxidation to elemental I2 (80) ... 

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