Base Hydrolysis of Chromium III Complexes

Loss of the first NCS ion from [Cr(NCS)6] in the presence of base is independent of the concentrations of OH and NCS . From the rate and activation parameters an 1 mechanism is proposed.  

The base hydrolysis of [Cr(dmso)6] has also been studied in aqueous NaOH. 0 exchange between the coordinated DMSO and OH was observed, and the slow step found to be loss of the first DMSO molecule. Base hydrolysis of [M(NH3)5NCSf ions (M = Co, Cr, Rh) has been studied in aqueous organic solvents (EtOH, Me2CO). fcobs = kX[OH] /(l + [OH ]), but with K ascribed to an ion-pair constant and not conjugate-base formation. The activation parameters were determined and the mechanism discussed. Data for the base hydrolysis of [CrX2L] ions (L = 2,3,2- or 3,2,3-tet) were considered previously (Table 6.3), as were results for the base-catalyzed oligomerization of [Cr(H20)6] (Reference 49) and the photolysis of [Cr(phen)3] under basic conditions (Reference 69). 

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).  

The trans-cis isomerization of K[Cr(ox)2(OH2)2] ion has been examined in aqueous/organic solvents (EtOH, dioxane, Mc2CO). The first-order rate constant decreases with increasing concentration of organic solvent. Both chelate-ring-opening and twist mechanisms are involved. This reaction is also catalyzed by ion pairing with Mg ions.  

From studies of the inversion reaction of A-(+)589-[Cr(trien)(ox)] in hydrochloric acid, to give A-(-)589-cis-a-[Cr(trien)Cl2], the oxalato complex is now believed to have the cis-jS configuration. This is a rare example of an acid-catalyzed isomerization of this type.  

The 5n1CB mechanism is favored for the base hydrolysis of trans-[Cr(L)Cl2r ion (L = C-meso-5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazacyclotetradecane, teta). For loss of the first CL ion, rate = 

AS value fits with the expectation of an 5n1CB mechanism. Comparison of rrans-[Cr(teta)Cl2] with rrans-[Cr(cyclam)Cl2] shows steric acceleration by C-methyl substitution in line with an 5n1CB mechanism. Loss of the second CF ion from the teta complex occurs at a similar rate to loss of the first CF, and initial rates were used in the kinetic analysis of the first stage. 

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A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

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. 

Hydrolysis of ammonia or amines is often observed, but only in a few cases have such reactions proved to be useful synthetically. Base hydrolysis (aqueous NH3) of the so-called rhodo ion, (NH3)5Cr(OH)-Cr(NH3)55 +, yields the so-called cis hydroxo erythro ion, cis-(NH3)5-Cr(OH)Cr(NH3)4(OH)4+, and both this ion and its corresponding acid form, cis aqua erythro have been isolated as salts (227, 252, 253). The hydrolysis is complete within minutes, and unlike the hydrolysis of many other ammine chromium(III) complexes, is quite a clean reaction, at least in solutions of moderate alkalinity (225). The corresponding trans aqua isomer has been prepared by heating the solid... 

A considerable amount of kinetic and mechanistic data has been reported for the base hydrolysis of cis- and trans-dihalobis(ethylenediamine)-cobalt(III) cations/ On the other hand, the only chromium(III) analogs which have been studied are cis- and trans-dichlorobis(ethylenediamine)-chromium(III) cations/ The present study was undertaken to gain additional base hydrolysis data for dihalobis(ethylenediamine)chromium(III) complexes, which will permit a more meaningful comparison with their cobalt(III) analogs. 

The hydrolysis of 1-acetylimidazole has been studied in the presence of hydroxo-cobalt(lll) and -chromium(III) complexes, for example [Co(NH3)5(OH)]. Catalysis is specific rather than general acid-base, implying strong complex-1-acetylimidazole interaction, presumably complex formation. This result is used to implicate direct involvement of Zn(OH)j in the mechanism of action of carbonic anhydrase. ... 

Observations on the reaction of ethylmalonatopentaamminecobalt(III) with Cr(H20)g + introduce a new element of interest. The chromium(III) products are the chelated malonate (67%) with a corresponding amount of free alcohol and the monodentate ester complex (33%). Since ester hydrolysis in the latter species is slow, we conclude that hydrogen results from the ester in the chelate form. Again, since ring closure of the monodentate product complex is slow, chelation must have occurred before Cr is oxidized to Cr . It is possible that formation of the chelate as primary product is complete, and that this product reacts in part to yield the monodentate product before hydrolysis occurs. Activation based on electron transfer to trap a function which is sensitive to a substitution-inert metal ion acting as a Lewis acid could presumably be extended to other more interesting situations. 

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