Chelate ring opening

As intracellular reduction mechanisms are important for Pt(II) and Pt(IV) drugs alike, the chemical reduction of cis,trans-[Pt(en)I2(OH)2] (3) by GSH was further explored under biologically relevant conditions (24), which led to the unexpected detection of a long-lived chelate-ring-opened Pt(II) complex capable of forming DNA-Pt adducts. 

The drastic drop in the rate of epimerization of ortho-substituted derivatives compared to the unsubstituted or para-substituted complexes is ascribed to the steric effect of the ortho substituents. In an intramolecular isomerization, steric hindrance is expected to increase with the size of the ortho substituents (//, 92). For a dissociation reaction, or for chelate ring opening, a steric acceleration should be observed if the ortho substituents are bulkier. These substituent effects therefore support an intramolecular formulation of the epimerization reaction in Scheme 16. 

Rate constants for chelate ring-opening in four-coordinated square-planar cf-platinum complexes [ 1,2-bis(phenylsulfinyl)ethane]dichloroplatinum(II) by a series of pyridines show from a negative deviation in a Bronsted plot (a = 0.53) an estimated rate decrease of steric origin of 200 for the 2-picoline (81IC71) (Scheme 33). 

Werner 50) and his students. He was the first to recognize that racemiza-tion of complexes such as M(AA)3, where AA is a bidentate, can take place by a chelate ring opening-closing process. Such a mechanism and two others are shown in Figure 9. These are called intramolecular processes, because racemization takes place without the complete departure of a ligand from the first coordination sphere. 

Figure 9. Intramolecular racemization of [M (A A) 3] by the following mechanisms (A) chelate ring opening-closing) B) rhombic twist) and (C) trigonal...

Both monodentate and chelated oxalato complexes undergo H+- and OH -catalyzed exchange of O atoms. For [Co(OC2 03/H)(NH3)5] this is faster (H" ") or comparable (OH ) with hydrolysis of the complex, while for [ 0(0262 02)(en)2] the two exocyclic O atoms exchange much faster than chelate ring opening. Some O-exchange data are given in (Table 62). 

Multidentate Leaving Groups.—Rates of aquation of [Co(C03)(LL)2]+, where LL = en, pn, tn, or (NHa)2, are strongly acid-dependent, whereas those of the unidentate carbonate complex [Co(C03)(NH3)5]+ are not. The mechanism of aquation of the chelate complex is thought to involve rate-determining chelate ring opening assisted by water or protonated water molecules. ... 

Individual steps in chelate ring opening can be followed kinetically during base hydrolysis of [Co en2C03]+, in contrast to acid aquation where only one stage can be monitored kinetically. Thus, while the intermediate formation of a unidentate carbonate complex in acid aquation is a postulate, its formation in alkaline solution is kinetically proven. Further discussion on mechanisms of reactions of carbonato complexes will be found in the kinetic section of a general review on the chemistry of these complexes. ... 

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