Chemical reactivity ligand substitution

Major emphasis is placed on the reactions of metal complexes in solution undergoing either inner-sphere ligand substitution or electron transfer to and from the metal center. Such studies relate to the important selective role of metal catalysts in many areas of enzymatic, commercial, and modem synthetic chemistry. Clearly, this field has now matured to the point where basic theoretical considerations, although incomplete, can provide a logical framework for understanding the chemical reactivity of such systems and stimulate the investigation of (1) new and unique reaction pathways, (2) modified reagents, and (3) unorthodox matrices. 

The effect of the polymer backbone on the intrinsic chemical reactivity of metal complexes has been studied in aqueous solution and in Nafion (perfluorocarbon sulfonic acid) film 44). Using a model catalyst-substrate system, the independent kinetic effects of reaction site homogeneity, substrate diffusion into the polymer film, and changes on activation parameters have been addressed. The ligand substitution reaction (6), was chosen for this purpose (Py = pyridine and its derivatives). 

While certain reactions of complexes which contain the M2L8 skeleton, and M-M quadruple or triple bonds (i.e. molecules of classes (a) and (B), vide supra), resemble those of dimers of classes (C) and (D), other aspects of their chemical reactivity are noticeably different. They resemble dimers (c) and (D) in undergoing ligand substitution reactions(2.), Lewis base associations (particularly in the case of carboxylate-bridged dimers of the type M2(02CR)u)( ) and dimer to cluster transformations, recent examples of the latter being M2 -----M3 (triangular)(15., 16),... 

Cu-ATSM is selectively trapped in highly reducing hypoxic tissue but not in less reducing normal tissue. On the other hand, Cu-PTSM is reduced in both normoxic and hypoxic tissue. Hypoxia selectivity has been correlated to the reduction potential of the molecule that is in turn related to the alkyl group substitution. The reduction potential of Cu-ATSM is about 100 mV lower than that of Cu-PTSM. Density Functional Theory and chemical reactivity studies attempt to correlate the structure and redox properties of bis(thiosemicarbazones) with the characteristics of the LUMOs (metal or ligand based) and chemical reactivity. The fast and selective uptake of jjj hypoxic... 

The fundamental reaction in the hydrolysis of metal alkoxides is a nucleophilic substitution (Sn ) that involves a nucleophilic addition of OH group onto the electrophilic metal center (M ). As a result, the coordination number (CN) of the metal atom is increased in the transition state, which leads to the migration of proton toward an alkoxy group, consequently knocking off the protonated ROH ligand (Figure 12). The chemical reactivity of metal alkoxides toward nucleophilic reactions mainly depends on the strength of the nucleophile, the electrophilic character of the metal atom, and its ability to increase the CN. 

---