Transition metals linkage isomerism

There is considerable and widespread interest in the metal complexes of these anions and current research topics comprise for example (i) the spectroscopic study of the binding in these anions (linkage isomerism) and their complexes, (ii) the synthesis of regular polymers of their transition metal complexes and study of the semiconducting properties of these polymers, (iii) the use of the pseudohalides in pharmacological (e.g. low toxicity of —SCN) and biochemical studies (easy complexation of SCN- to metals), and (iv) the use of the activation of these triatomic anions by coordination to metals for their selective conversion in organic synthesis. 

Isomeric (s-cis- and (i-fra/w-V-conjugated diene)zirconocene and -haf-nocene complexes exhibit pronounced differences in their characteristic structural data as well as their spectroscopic features. These differences exceed by far the consequences expected to arise simply from the presence of conformational isomers of the 1,3-diene unit. While (f-rra/u-butadiene)-zirconocene (3a) shows a behavior similar to a transition metal olefin TT-complex, the (.r-cu-diene)ZrCp2 isomer 5a exhibits a pronounced alkylmetal character (23, 45). Typical features are best represented by a tr, 7T-type structure for 5 (55). However, the distinctly different bonding situation of the butadiene Tr-system/bent-metallocene linkage is not only reflected in differences in physical data between the dienemetallocene isomers 3 and 5, but also gives rise to markedly different chemical behavior. Three examples of this are discussed in this section the reactions of the 3/5 isomeric mbcture with carbon monoxide, ethylene, and organic carbonyl compounds. 

In many transition metal complexes, geometrical and linkage isomerization reactions involve ligand substitution processes. One example of a photoin-duced linkage isomerization reaction, namely, the isomerization from an S-to an O-bonded sulfinato complex, Co(en)2(S02CH2CH2NH2)2+, has been studied as a function of pressure. The O-bonded product undergoes a thermal back reaction to the S-bonded complex and this system, shown in... 

The factors that affect linkage isomerization of ambidentate ligand complexes of transition metals have previously been reviewed by Baluhura and Lewis (72). We will summarize some of these factors. 

Although PI is a plausible intermediate on the reaction path in such reactions, it has not been detected directly by experiment and, hence, its proposed existence remains largely speculative. However, indirect proof for the presence of PI on the reaction path comes from the observed isomerization between 3, 5 -Unkage and 2, 5 -linkage only under acidic pH where PI can exist in the monoanionic form (PIH). Thus, an alternative mechanism (Scheme 2.26) for the metal-ion-assisted hydroxide-ion- and general base ( B)-catalyzed hydrolysis of RNA and RNA-model compounds cannot be completely ruled out if pentacoordinated intermediate (PIH) does exist on the reaction path. Nearly 10 -fold enhanced catalytic effects of Co " complexes are attributed to the occurrence of intramolecular electrophilic catalysis as shown in the transition state TS,g. 

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