Substitution reactions dissociative

There is also evidence supporting a as effect on the rate of square planar substitution reactions, although it is not quite as dramatic. However, steric factors can be very important in the c/s position. Steric congestion enhances the rate of a dissociative substitution reaction and slows the rate of an associative one. Because the bulky ligands are closer to each other in the c/s position than in the trans position. 

Dissociative substitution reactions involving low-spin /m 5-[Fe(cyclam)-(NCS)X]+ ions were considered in Chapter 5 (ref. 130). 

Table 5.1. Trends for associative and dissociative substitution reactions.

Reaction coordinate diagram for a dissociative substitution reaction. 

Relationship between the enthalpy of activation and enthalpy of bond cleavage in a dissociative substitution reaction. 

As a result, activation enthalpies of many dissociative substitution reactions of 18-electron complexes are relatively high and close to the M-L BDE. The weak M-CO bond energy of Ni(CO) (25 2 kcal /mol) allows this complex to react rapidly at room temperature, but the higher M-L bond energies of the olefin in CpRhCethylene) and CO in Cr(CO)g (31 kcal/ mol and 37 2 kcal/mol, respectively) cause the reactions of these complexes to require temperatures of > 100 °C and 80-140 °C, respectively, to occur at reasonable rates. 

Dissociative substitution reactions, even in alkane solvents, occur through intermediates that typically contain coordinated solvent. For this reason and because of the intermediacy of alkane complexes in C-H activation, a number of studies have been conducted on the mechanism of the displacement of alkanes and even noble gases from the coordination sphere of a metal. A few of these studies are summarized in Scheme 5.5. 

I.6. Electronic Effect of Ancillary Uganda on ffte Rates of Dissociative Substitution Reactions—We Cis Effect... 

Ligands other than the departing carbon monoxide will influence the rates of substitution reactions. Good electron donating ligands such as amines and phosphines speed dissociative substitution reactions, because they compensate for the loss of electrons on the metal that occurs in the rate-determining step. This is an important factor to consider when one wants to increase the rate of ligand loss in a catalytic cycle. 

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