Chelate ring closure

A // = 40kJmol-1 AS = —204JK 1mol 1). The activation parameters indicate associative activation for both steps (212). Associative chelate ring closure here is consistent with the mechanistic pattern established in organometallic systems (213), where this process is associative for [M(CO)5(diimine)] where M = Mo or W (though dissociative for M = first-row Cr (214)) and the diimine does not carry bulky substituents. 

Kinetic Parameters for Chelate Ring Closure of [PtPh2(CO)-... 

Reviews " of pentacyanoferrate substitution kinetics have included a detailed consideration of high-pressure studies of thermal and photochemical substitution and electron transfer reactions of pentacyanoferrates-(II) and -(III). Photochemical activation can result in the loss of L or of CN . The best way to study the latter is through photochemical chelate ring closure in a pentacyanoferrate complex of a potentially bidentate ligand LL [Fe(CN)5(TL)]" rFe(CI 4(LL)] " +... 

The chelate ring closure described in equation (42)165,166 would be represented by a more concerted version of the general type shown by equation (35). The final product (63) has also been treated with a,a -dibromo-o-xylene to produce the related macrocycle, via its complex.165... 

TABLE 13 Activation Volumes for Chelate Ring Closure in M(CO)sL Complexes in Toluene... 

The following chelate ring-closure reaction is an example of intramolecular substitution in a square-planar complex ... 

This equation is consistent with the experimental observations a zero intercept is required but does not imply that k 2 is zero. This rate law corresponds to the condition of a rapid pre-equilibrium formation of the monodentate species, k lk, followed by rate-controlling chelate ring closure. The values originally associated with are actually k. k ky. Since these are numerically similar to rate constants observed for monodentate systems, it would appear that the ratio 35/ 34 is of the order of magnitude of 1. 

The pH dependence of the rates of complex formation between Ni + and picolinic acid (pyridine-2-carboxylic acid) and fusaric acid (5-butylpicolinic acid) are not linear functions of but can be accounted for by a reaction scheme in which initial complexing occurs at the carboxylate group and chelate ring closure is an important kinetic factor (Scheme 2). If the rate law is expiessed in the form — d[Ni2+]/d/ = A obs[Ni][total ligand... 

Clearer evidence for two isomers when X = Br comes from recent C n.m.r. studies in which the major component is assigned the unexpected axial structure (Br out of the N—Co—N plane) in contrast to the more usual equatorial configuration. Further studies which sometimes involve chelate ring closure are reported for the following systems ... 

A dissociative mechanism is proposed for the loss of chloride ion from mer-[RhL L Cls] resulting in chelate ring closure ... 

Reaction (52) involves the loss of one unidentate 0-bonded amino-acid anion, and reaction (53) is a process of chelate ring closure and topological shift to give the... 

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