Aquation Chromium m Complexes

Chromium(ni) continues to be the second most widely studied metal ion after cobalt(m). The question of dissociative (/a) or associative (/a) mechanisms for substitutions at chromium(m) has been clarified somewhat by comparing data for the formation and dissociation of the [Cr(H20)5X] + and [Cr(NH8)5X] + ions (X = unidentate leaving anion). Recent studies indicate that an associative mechanism is important for aqua-chromium(m) complexes, but for the [Cr(NHa)6X] + ions a dissociative-interchange mechanism is favoured. A summary of kinetics and thermodynamic data for the formation and aquation of [Cr(NH3)5X] + ions is given in Table 12. 

Several general features of substitution at chromium(m) have already been mentioned in the first section of this chapter. The use of activation volumes and of transition enthalpies in the assignment of substitution mechanisms is discussed there, as is evidence which suggests the operation of a mechanism with some associative character at certain chromium(m) complexes, but of a D [. NlClim)] mechanism at others. 

---