Transition metal chelate compounds

The jS-diketones are a versatile class of ligand with many known modes of coordination, some of which are shown in Figure 3. 0,0 -chelation (11) of the monoanion is by far the most common mode, being found for almost all of the metallic and metalloid elements. Compounds of the stoichiometry M(dike)3 and M(dike)2 are the most common. The former generally have an almost octahedral distribution of the six oxygen atoms when M is a main group or transition metal. These compounds have been used extensively in studies of the mechanism of the racemization of trischelate complexes. 

The five- and six-membered transition metal chelate rings contain examples of compounds which have been most extensively studied, theoretically as well as experimentally. The tris-(gIyoxalbis-N-me thy limine )iron (II) chelate 23 synthesized by Krumholz 68> is the structurally simplest example of a vast number of intensely coloured chelates containing different 1,2-diimine ligand systems which have been recently reviewed 69>. (pd)n -Bonding was thought to be so important for this type of chelates that it could explain their properties by an aromatic 70> or quasiaromatic 71-72) behaviour. These types of chelates will be considered specifically in Chapt. V. 

Dithiocarbamates form a wide range of complexes with transition metals, and compounds of other metals such as Sn and of non-metals such as Te are also known. The CS2 group in dithiocarbamates as well as in xanthates, thioxanthates, and thiocarbonates is usually chelated as in (10-XIX), but a few cases of monodentate dithiocarbamates are known.39... 

Full details have appeared of the quenching of 2 by transition-metal chelates,118 and a comparison of their quenching properties towards 102 and triplet carbonyl compounds has been published.117 The azomethine dyes, of general formula (55), form a class of x02 quenchers which, unusually, do not... 

Transition metal hydrides. (J. P. McCue, Coordination Chem. Rev., 1973, 10, 265). Transition-metal cluster compounds. (R. B. King, Progr. Inorg. Chem., 1972,15, 287). Stereochemistry of bis-chelate metal(ii) complexes. (R. H. Holm and M. J. O Connor, Progr. Inorg. Chem., 1972, 14, 241). 

Dioxygen complexes of the transition metal chelates have received considerable attention especially as model compounds in the study of the reversible oxygenation mechanisms in biological systems. The study of the reversible oxygen-carriers is of very importance in understanding certain aspects of bioinorganic chemistry and to obtain ultrapure oxygen by unconventional methods Cl-4j. ... 

Technetium-99m coordination compounds are used very widely as noniavasive imaging tools (35) (see Imaging technology Radioactive tracers). Different coordination species concentrate ia different organs. Several of the [Tc O(chelate)2] types have been used. In fact, the large majority of nuclear medicine scans ia the United States are of technetium-99m complexes. Moreover, chiral transition-metal complexes have been used to probe nucleic acid stmcture (see Nucleic acids). For example, the two chiral isomers of tris(1,10-phenanthroline)mthenium (IT) [24162-09-2] (14) iateract differentiy with DNA. These compounds are enantioselective and provide an addition tool for DNA stmctural iaterpretation (36). 

Of the ten trace elements known to be essential to human nutrition, seven are transition metals. For the most part, transition metals in biochemical compounds are present as complex ions, chelated by organic ligands. You will recall (Chapter 15) that hemoglobin has such a structure with Fe2+ as the central ion of the complex. The Co3+ ion... 

Transition metal polysulphides coordination compounds with purely inorganic chelate ligands. M. Draganjac andT. B. Rauchfuss, Angew. Chem., Int. Ed. Engl., 1985, 24, 742 (129). 

The first complexes of a-keto ylides and group 5 early transition metals have only recently been obtained by reaction of Nb(III) derivatives [[NbCl3(dme) (R C=CR")] with 25 (R = thiazolyl) (Scheme 16). The chelation of the ylide occurs through an N,0-coordination to the metal center and in presence of MeLi a deprotonation of a phenyl ring takes place with the loss of alkyne, leading to the formation of a new orfho-metallated binuclear compound 32. The two ylides involved in the complexation behave as tridentate anionic ligands and are mutually in a trans disposition in order to minimize the steric hindrance [71,72]. Another binuclear niobium complex 33 has been obtained from 25 (R = Me, Ph) with this time an 0-coordinated a-keto ylide [68]. 

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