Anation reactions kinetics

IS further extended by the replacement of X by other anionic or neutral ligands. The inertness of the compounds makes such substitution reactions slow (taking hours or days to attain equilibrium) and, being therefore amenable to examination by conventional analytical techniques, they have provided a continuing focus for kinetic studies. The forward (aquation) and backward (anation) reactions of the pentaammines ... 

Table 20.2 Kinetic Data for the Anation Reactions of Aqua Complexes3. ...

In proposed mechanism I, the loss of water from the complex is the rate-determining step, but removal of water from the coordination sphere of the metal ion should be independent of the nature of the anion that is not part of the coordination sphere of the metal ion. On the other hand, if mechanism II is correct, the entry of X into the coordination sphere of the metal would be dependent on the nature of the anion, because different anions would be expected to enter the coordination sphere at different rates. Because there is an observed anion effect, it was concluded that the anation reaction must be an Sn2 process. However, it is not clear how a process can be "second-order" when both the complex cation and the anion are parts of the same formula. As discussed in Chapter 8, it is not always appropriate to try to model reactions in solids by the same kinetic schemes that apply to reactions in solutions. 

As the above discussion indicates, assigning mechanisms to simple anation reactions of transition metal complexes is not simple. The situation becomes even more difficult for a complex enzyme system containing a metal cofactor at an active site. Methods developed to study the kinetics of enzymatic reactions according to the Michaelis-Menten model will be discussed in Section 2.2.4. 

The aquation of the various CoCCN -3 complexes must occur by a reaction path which is merely the reverse of that followed in the anation. If the proposed mechanism for the anation reaction is valid, the reverse of Reactions 1 and 2 may be used to describe the equation. In any given experiment the rate of approach to equilibrium may be characterized by a first-order rate constant k which is related to the other kinetic parameters by Equation 3. When krfa/kz k (X ), as it is in... 

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. 

Kinetically, there are inherent difficulties in distinguishing the three mechanisms. As an illustration, is the anation reactions of the inert (d low-spin) complexes of cobalt(III), as typified by... 

Part of the misconception regarding the kinetics of the deaquation-anation reaction stems from the fact that only a Hmited analysis of the data was performed. To be complete, the data should be analyzed using all of the rate laws shown in Table 7.2. A more recent study of this process by Hamilton and House was completed in which the reaction was studied by means of mass loss as the H2O is driven off. Figure 7.4 shows typical rate plots that were obtained for the process when carried out at several constant temperatures. 

Hamilton, D. G., House, J. E. (1994). Transition Met. Chem. 19, 521. The kinetic study on the deaquation-anation reaction of aquapentamminecobalt(III) chloride. 

Kinetic studies on palladium complexes of dien, and of its more bulky N-substituted forms, continue to furnish valuable mechanistic information. Iodide anation reactions of [Pd(OH2)(R5-dien)] (R = H, Me, or Et) at pH values between 7 and 12 throw light on previously reported deviations in kinetic data above pH The value of kobs increases with increasing [I ], and decreases with increasing steric bulk of Rs-dien and increasing pH. For pH values up to 9, plots of kobs against [I ] are linear and pass through the origin, and can be accounted for by the reactions of Scheme 2 where [Pd(OH)(R5-dien)] is kinetically inert. 

Substitution at platinum(ii) may be involved when Pt complexes are used to catalyse substitution at platinum(iv). Some new views on the role of the platinum(ii) species in such reactions have been offered. The notion of direct relevance to this chapter is that the primary step is formation of a dinuclear intermediate directly from the simple hydrated platinum(ii) complex and the platinum(iv) complex. Aquation, Solvolysis, and Anation. The kinetics of aquation of [PtClJ over the range 50—70 °C are consistent with data published earlier for aquation at lower... 

Table 5.2 gives some kinetic and thermodynamic data related to this reaction. First note that the rate constants, unlike those for the anation reactions just discussed, now do seem to vary quite significantly with various ligands, L . Such a variation is consistent with a rate-determining step in which M-L bonds of varying strength are broken. 

The novel feature of the reaction of [Ru(NH3)6(OH2)] + with the N-methylpyrazinium (nmpz ) ion is the employment of a cationic entering group. The kinetic pattern is complicated by the existence of a parallel redox reaction, but the anation reaction appears to involve two steps ... 

Kinetic studies of aquation of dinuclear [ traras-PtCl(NH3)2 2 (p-NH2(CH2)6NH2)]2+ established rate constants for the loss of the first and second chloride ligands (7.9 x 10-5 and 10.6 x 10-4s-1), and for the reverse anations (1.2 and 1.5M-1s-1). Reactivities here are very similar to those in analogous mononuclear systems [Pt(amine)3Cl]+ (204). A kinetic and equilibrium study of axial ligand substitution reactions... 

The relative reaction rates and the stability of the aquo complex make it possible to identify the aquo complex as an intermediate and study the individual acts separately. However, if the solvento complex were less stable and the anation rate much faster than the solvolysis, it would not be possible to observe this intermediate, and the process would be kinetically indistinguishable from a unimolecular dissociative process. Both processes would exhibit overall first-order kinetics and the usual mass-law retardation and other competitive phenomena characteristic of an extremely reactive intermediate. 

H20 Reaction with oxalate, cis/trans synthesis and kinetics of anation with oxalate, nitrate catalysis 1... 

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