Aquation and Solvolysis of Chromium III Complexes

J K mol A in cm mol visible absorption spectral maxima rnax) in nm, and molar extinction coefficients (e) in M cm  

Data for these systems are presented in Table 6.2. Complexes with weakly acidic leaving groups often have an acid-dependent path with rate law (1). 

Occasionally the thermal aquation of [CrX(NH3)5] systems is complicated by NH3 loss, and this seems especially prevalent when X is large.  

The mechanistic interpretation of the pressure dependence of the aquation rate is considerably simplified by using neutral ligands as leaving groups.  

Substitution Reactions of Inert Metal Complexes—6 and Above 

The [Cr(H20)5CH2l] ion is postulated to react via a carbanion-type transition state in which a proton is transferred from the incoming water molecule to the leaving CH2I group to give Mel and [Cr(H20)50H], which rapidly protonates. 

In the case of [Cr(H20)5CHX2] ions, a simple acid-independent first-order rate law is observed again, except when X = I for which the rate law is rate = (iko + [Cr(H20)5CHIi ] at 298 K, 10% = 2.0 s , = 

Two studies involving mixed DMSO-H2O solvates of Cr(III) have been reported.Continuing an investigation of the labilization of Cr(III) complexes by coordinated oxyanions, the reaction of the nitratopenta-aquochromium(III) ion in acidic water-DMSO mixtures has been reported. Up to five DMSO mol- 

With this scheme, ka = Kik2/(l + A 2/ 3), and r = K1K2IC3, s = K1K2, Indirect support for the proposed mechanism comes from the synthesis, in low yields, of [CrClBr] ion from the reaction of Cr with aqueous bromine in 0.5 mol dm [HCl]. 

With neutral leaving groups, the average A V is approximately -6 cm mol for Cr(III) and approximately +2 cm mol for Co(III) with triflate as the leaving group, the corresponding data are about -9 and about -3, respectively. 

The hydrolysis rates and equilibrium measurements (Table 6.1) for [CrX (H20)6 ] (X = Cl, Br n = 1,2) in acidic solution, Eq. (1), provide a unique and historical study, in so far as three generations of Bjerrum have now investigated these systems. The data in Table 6.1 show that A i(X = Br) is inversely proportional to [HBr] and not [H ] as originally stated. 

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Whereas the assignment of mechanism to spontaneous thermal aquations may at times be uncertain, the mechanism of metal-ion-catalysed aquation of halide complexes of cobalt(iii), chromium(ni), and similar cations is unlikely to be other than dissociative as far as the metal(m) centre is concerned. In Volume 2 of this Report it was mentioned that the catalytic effect of metal ions on solvolysis rates of t-butyl halides could be correlated with the stability constants of the respective metal-halide complex formed. Such a correlation is now reported for metal-ion catalysis of aquation of halide complexes of cobalt(m), chromium(m), and rhodium(m). Indeed this correlation is sufficiently general as to embrace such catalysts as H+ and HgCl+ as well as metal ions such as Hg + and A linear free-energy (AG vs. AG°) correlation... 

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