Second 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. 

Other cases in which second-order kinetics seemed to require an associative mechanism have subsequently been found to have a conjugate base mechanism (called S ICB, for substitution, nucleophilic, unimolecular, conjugate base in Ingold s notation ). These reactions depend on amine, ammine, or aqua ligands that can lose protons to form amido or hydroxo species that are then more likely to lose one of the other ligands. If the structure allows it, the ligand Irons to the amido or hydroxo group is frequently the one lost. 

The 19-electron complexes also undergo very fast ligand-substitution reactions according to the same associative mechanism (second-order kinetic low), because they are in very fast pre-equilibrium (eventually intramolecular) with the 17-electron complexes. In the intramolecular case, the 17- and 19-electron forms can even sometimes be mesomer forms. 

The kinetics of water substitution of some alkylaquocobaloximes [RCo(dmgH)2(H20)] (R = Me, CHjCl, CF3CH2) has been investigated in aqueous solution. Nucleophiles included NH3, imidazole, morpholine, and SCN . Evidence for pre-equilibrium association of the alkyl-aquocobaloxime with some ligands (NH3, imidazole, morpholine) was obtained using high nucleophile concentrations. Normally, strictly second-order kinetics (first order in the complex and first order in the nucleophile) is observed for such reactions. 

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. 

A completely empirical LFER can also be constructed with recourse only to kinetic data. This has been the case in the setting up of a scale of nucleophilic power for ligands substituting in square-planar complexes based on the Swain-Scott approach. The second-order rate constants Ay for reactions in MeOH of nucleophiles Y with tra 5-Pt(py)2Cl2, chosen as the standard substrate... 

In contrast to the facile reduction of aqueous V(III) (—0.26 V versus NHE) [23, 24], coordination of anionic polydentate ligands decreases the reduction potential dramatically. The reduction of the seven-coordinate capped-octahedral [23] [V(EDTA)(H20)] complex = —1.440 V versus Cp2Fe/H20) has been studied extensively [25,26]. The redox reaction shows moderately slow electron-transfer kinetics, but is independent of pH in the range from 5.0 to 9.0, with no follow-up reactions, a feature that reflects the substitutional inertness of both oxidation states. In the presence of nitrate ion, reduction of [V(EDTA) (H20)] results in electrocatalytic regeneration of this V(III) complex. The mechanism was found to consist of two second-order pathways - a major pathway due to oxidation of V(II) by nitrate, and a minor pathway which is second order in nitrate. This mechanism is different from the comproportionation observed during... 

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