Coordination complexes factors affecting stability

The purpose of this chapter is to explore the properties and reactions of various Pt-nucleobase complexes. After a short description of various binding modes, attention will be paid on the effects of coordinated platinum. Topics include, e.g., isomerization, thermodynamic stability, and solvolyt-ic reactions of Pt-nucleobase complexes. Finally, factors affecting the mechanism and kinetics of substitution reactions by various nucleophiles will be discussed. 

There are many problems still to be solved for this quite simple reaction. For the reaction to be reversible and fast the alkyls must be unstable, but the criteria for stability of different types of alkyls are not fully established. It must be noted that the alkyl group occupies only one coordination site, whereas in the transition state and in the hydridoalkene complex, two sites must be available, so that clearly a prime factor is coordinative unsaturation which allows decomposition of the alkyl by elimination of alkene. The factors affecting the direction of addition, or the selectivity in elimination from secondary alkyls to give cis- or inm -isomers, are similarly not well understood. For the first, it appears that the greater the polarity of the M—H bond in the direction Ma —H[Pg.787]

The formation of a coordinate bond is the result of the donation and acceptance of a pair of electrons. This in itself suggests that if a specific electron donor interacts with a series of metal ions (electron acceptors) there will be some variation in the stability of the coordinate bonds depending on the acidity of the metal ion. Conversely, if a specific metal ion is considered, there will be a difference in stability of the complexes formed with a series of electron pair donors (ligands). In fact, there are several factors that affect the stability of complexes formed between metal ions and ligands, and some of them will now be described. 

Fig. 5.20. Modes of coordination of transition metal ions with /3-lactam antibiotics. Complex A In penicillins, the metal ion coordinates with the carboxylate group and the /3-lactam N-atom. This complex stabilizes the tetrahedral intermediate and facilitates the attack of HO-ions from the bulk solution. Complex B In benzylpenicillin Cu11 binds to the deprotonated N-atom of the amide side chain. The hydrolysis involves an intramolecular attack by a Cu-coordinated HO- species on the carbonyl group. Complex C In cephalosporins, coordination of the metal ion is by the carbonyl O-atom and the carboxylate group. Because the transition state is less stabilized than in A, the acceleration factor of metal ions for the hydrolysis of cephalosporins is lower than for penicillins. Complex D /3-Lactams with a basic side chain bind the metal ion to the carbonyl and the amino group in their side chain. This binding mode does not stabilize the tetrahedral transition complex and, therefore, does not affect the rate of... 

There are factors other than olefin basicity and the steric effects of substituent groups on the olefin which can affect the stability of the silver ion complex. These include the energy required to displace solvate molecules from the coordination sphere of the metal ion and the degree of association between the cations and anions, especially in concentrated solutions or in solid salts. 

Metal coordination is essential to the function of electron transfer in MCOs. Accordingly, metal binding and metal complex stability during protein synthesis and subsequent purification is a critical factor [61 ]. A few questions outline the metal complex issues How efficient is copper loading during protein expression When the metal is in complex with the protein, how does its coordination affect the purification of the enzyme and its stability How stable are the copper centers during purification, storage, and when used in fuel cell applications ... 

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