Chromium anation reactions

Kinetic data for the deaquation-anation reactions [33] of a range of mixed amine coordination compound salts of chromium(III) were discussed with a view to determining the mechanisms of the reactions observed. From the results it was concluded that behaviour is influenced by the available free space in the crystal structure and that dehydration, accompanied by isomerization, involves the simultaneous rupture of Cr-N (organic ligand) and Cr-0 (water) bonds. 

The solvolysis of hexaaquachromium(HI) in DMSO proceeds via the series of complexes [Cr(DMS0)n(H20)6 ]3+ (n = 1 to 6).945,946 The anation and solvolysis of several chromium(III) complexes in DMSO have been studied 947,948 in general the reactions proceed by /d mechanisms. A direct electrochemical synthesis of [Cr(DMSO)6][BF4]3 has been reported.949 A comprehensive review of metal ion complexation by DMSO contains an extensive section on chromium(III).950... 

Although all of these examples involve cobalt(III) complexes, reactions involving other metal ions could be cited as examples. This paper will deal mainly with the complexes of cobalt(III) and chromium(III) and will be limited to the anation, racemization, and geometric isomerization processes. Linkage isomerization will not be discussed since such reactions are fairly familiar and have been recently reviewed elsewhere. " Ligand exchange will be discussed only as it is encountered in racemization and geometric isomerization. 

Stereochemical and kinetic studies of the stepwise aquation of the [Cr(CN)6] and [Cr(CN)5(NO)] anions have indicated that the cyano-ligand has a strong rranj -labilizing effect in this type of complex. Hence it seemed an attractive idea to study this postulated effect in the simplest member of this group, the [Cr(OH2)5(CN)] + cation. Unfortunately accurate kinetic studies of substitution reactions of this complex were ruled out by the observation that aquation and anation processes occur at comparable rates.The strong trans effect of iodide complexed to chromium(m) is mentioned in the section on mixed and non-aqueous solvents below. 

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