First order kinetics ligand substitution reactions

The products of this first stage of the reaction, cis Mo or Cr(CO)4LX and cis Mn or Re(CO)4LX undergo further substitution. The Mn, Re compounds behave according to first-order kinetics while the Cr, Mo compounds show second-order kinetics to form trans Cr(CO)4L2 or cwMo(CO)4L2, rate coefficients increasing this time as expected, with the basicity of the entering ligand L. 

The mechanism of ligand substitution reactions in the carbyne complexes /rar7.v-M(CR)X(CO)4 (M = Cr, W R = Me, Ph, NEtj X = Cl, Br, 1, SePh) was investigated by H, Fischer and co-workers (JOO). The influence of the metal center, the trans ligand, and the carbyne substituent on the M—CO dissociation step was determined. The reactions with PPhj in 1,1,2-trichloroethane [Eq. (62)] all follow first-order kinetics, with activa-... 

Although first-order kinetics in both and are commonly observed, the majority of electrochemical reactions occur in more than one step -. Thus, e.g., for metal electrodeposition, electron and phase transfer occur in two distinct steps. As for homogeneous redox processes, various chemical steps (e.g., ligand substitution) either preceding or following the electron-transfer step often are encountered. Also, there is evidence that multielectron transfer occurs in microscopically separable one-electron steps. Even for one-electron transfer where both Yq and Y, are solution species, a separate adsorption step often precedes electron transfer. 

Both of these complexes are known to undergo thermal substitution near 100 C via clean first-order kinetics Q4. IS- Thus we performed the equilibrium experiments between 80 and 120 C. Since the equilibrium constant was expected to heavily favor the SBu2 complex 2 we ran the experiments with high concentrations of cw-cyclooctene, in either neat cw-cyclooctene (7.7 M) or 1.2 M solutions of cw-cyclooctene in heptane. The results obtained in both solvent systems were identical. Although differential vaporization of the ligands did not appear to be a problem, efforts were made to minimize the gas volume in the reaction vessel. 

For each of these reactions kinetic data were obtained. The reactions were first order in complex concentration, and zero order in isocyanide, as expected. The complex Ni(CNBu )4, and presumably other Ni(CNR)4 complexes as well, undergo ligand dissociation in solution. In benzene solution, a molecular weight determination for this compound gives a low value (110). This is in accord with the presumed mechanism of substitution. 

Lastly, it is appropriate to comment on the relationships between the intermediates seen in photochemical studies and possible reactive intermediates along the reaction coordinates of related thermal transformations. Earlier kinetics studies (] 3) of the reactions of Ru3(CO)i2 with various phosphorous ligands PR3 have found evidence for both first order and second order pathways leading to substitution plus some cluster fragmentation. The first order path was proposed to proceed via reversible CO dissociation to give an intermediate analogous to II. 

The results of the kinetic analysis for the investigated systems are summarized in Table 10.2, the substrate concentration used being the same for all trials. In the case of methyl- and cyclohexyl-substituted ligands the Michaelis constant is smaller than the initial substrate concentration of [S]o=0.06666 mol L-1 (Table 10.2). However, a description of the hydrogenations with other catalyst ligands as first-order reactions shows that in each of these cases the Michaelis constant must be much greater than the experimentally chosen substrate concentration. 

Platinum(IV) is kinetically inert, but substitution reactions are observed. Deceptively simple substitution reactions such as that in equation (554) do not proceed by a simple SN1 or 5 2 process. In almost all cases the reaction mechanism involves redox steps. The platinum(II)-catalyzed substitution of platinum(IV) is the common kind of redox reaction which leads to formal nucleophilic substitution of platinum(IV) complexes. In such cases substitution results from an atom-transfer redox reaction between the platinum(IV) complex and a five-coordinate adduct of the platinum(II) compound (Scheme 22). The platinum(II) complex can be added to the solution, or it may be present as an impurity, possibly being formed by a reductive elimination step. These reactions show characteristic third-order kinetics, first order each in the platinum(IV) complex, the entering ligand Y, and the platinum(II) complex. The pathway is catalytic in PtnL4, but a consequence of such a mechanism is the transfer of platinum between the catalyst and the substrate. 10 This premise has been verified using a 195Pt tracer.2011... 

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