Stereochemistry metal complexes

The notion of using molecules as storage centers for electronic memory is seductive. Use of molecules in memory clearly allows for extraordinary miniaturization, thereby permitting a high density of information storage. Because of their capacity to store multiple electrons (bits) and to exhibit diverse stereochemistries, metal complexes are of particular interest. One feature needed for a practical molecular memory is the ability of the redox functionahty to sustain multiple read-write cycles, that is, to withstand multiple oxidations and reductions without decomposition. Such a consideration is a concern for metal complexes that are typically more stable in one redox form compared to the others. 

In general, the Atlas covers the field of organic and organometallic (e.g. metallocene) stereochemistry. Metal complexes are not included they have been comprehensively covered by a recent monograph (C. J. Hawkips, Absolute Configuration of Metal Complexes (Interscience Monographs on Chemistry), Wiley-Interscience, 1971.)... 

The stereochemistry of metal complexes of nucleic acid constituents. D. J. Hodgson, Prog. Inorg. Chem., 1977, 23,211-254 (175). 

Recent aspects of the stereochemistry of Schiff-base metal complexes. S. Yamada, Coord. Chem. Rev., 1966,1, 415-437 (65). 

Site symmetry symbols, I, 128 Six-coordinate compounds stereochemistry, 1, 49-69 Six-membered rings metal complexes, 2, 79 Skeletal muscle sarcoplasmic reticulum calcium pump, 6, 565 Slags... 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Blauer G (1974) Optical Activity of Conjugated Proteins. 18 69-129 Bleijenberg KC (1980) Luminescence Properties of Uranate Centres in Solids. 42 97-128 Boca R, Breza M, Pelikan P (1989) Vibronic Interactions in the Stereochemistry of Metal Complexes 71 57-97... 

Recent reports on transition metal complexes of 2-heterocyclic thiosemicar-bazones suggest that stereochemistries adopted by these complexes often depend upon the anion of the metal salt used and the nature of the N-substituents. Further, as indicated previously, the charge on the ligand is dictated by the thione-thiol equilibrium which in turn is influenced by the solvent and pH of the preparative medium. Many of the reported complexes have been prepared in mixed aqueous solvents, often with bases added. However, there are few reports in which workers have varied the nature of their preparations to fully explore the potential diversity of these ligands. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Natural circular dichroism (optical activity). Although circular dichroism spectra are most difficult to interpret in terms of electronic structure and stereochemistry, they are so very sensitive to perturbations from the environment that they have provided useful ways of detecting changes in biopolymers and in complexes particularly those remote from the first co-ordination sphere of metal complexes, that are not readily apparent in the absorption spectrum (22). It is useful to distinguish between two origins of the rotational strength of absorption bands. 

The stereochemistry of 2 1 metal complex azo dyes is discussed fully in ref. 7 and is not repeated here). X-ray diffraction studies14 on such a 2 1 octahedral meridial nickel(II) complex azonaphthol dye (20) show that it is the /3-azo nitrogen atom that is coordinated to the nickel atom. It is very likely that the commercially important 2 1 Cr3+and Co3+ dye complexes bond in a similar way. 

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