Rare coordination chemistry

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

The choice of metal ion in this work is interesting since it has been known for a considerable time that Ag+ is a rare example of a d-block metal ion that does not disrupt the duplex DNA structure (172,173). Rationalization of this effect has tended to focus on the possible base-pair crosslinking due to the preferred linear coordination geometry of Ag1 ions (174). The importance of Ag+ DNA coordination chemistry to the procedure described is not clear. However, reports that other metal ions, e.g., Pdri (175), can be plated to DNA to fabricate metallic wires (Fig. 51) suggests that this may not be essential. 

While 61Ni Mossbauer spectroscopy has proven valuable in solid-state chemistry, its application to problems in coordination chemistry is very rare. The parent 61Co nucleus (/ /2 99 min) serves as the... 

These were found to have a particularly rich coordination chemistry, because the close proximity of the two gold atoms leads to significant transannular aurophilic interactions, facilitating, for example, the stoichiometric oxidation to compounds with a transannular Au-Au bond with the metal atoms in the rare Au(ii) state (Scheme >9).2,166,168... 

In inorganic and coordination chemistry, the Cu(II) state is the most abundant one, and is regarded as more stable than the Cu(II) state under normal conditions [32]. Although numerous examples of Cu(I) coordination complexes are known, their chemistry is rather limited and they are readily oxidized to Cu(II) species [32]. Of the common oxidation states, compounds derived from copper(III) are rare, with only 30-40 reported examples [32]. Despite the small number of isolated Cu(III) compounds, however, organocopper] 111) species have been proposed as important intermediates in copper-mediated organic reactions (Chapts. 4 and 10). 

The coordination numbers of metal ions range from I, as in ion pairs such as Na CI- in the vapor phase, to 12 in some mixed metal oxides. The lower limit, I. is barely within the realm of coordination chemistry, since the Na+CI km pair would not normally be considered a coordination compound, and there are few other examples. Likewise, the upper limit of 12 is not particularly important since it is rarely encountered in discrete molecules, and the treatment of solid crystal lattices such as hexagonal BaTiOj and perovskite1 as coordination compounds is not done frequently. The lowest and highest coordination numbers found in typical coordination compounds are 2 and 9 with the intermediate number 6 being the most important. 

Reported coordinating properties of aromatic hydroxy acids have been concerned mainly with salicylic acid (2) and substituted salicylic acids (12). Most published work refers to solution, where detailed characterization of the stereochemistry of the species formed is very rare. A series of recent publications by Lajunen et on the coordination chemistry of substituted hydroxynaphthoic... 

The above examples, which relate to but few of the many possible addition reagents in use, indicate that chemically the systems may be quite complex. The additives are certainly active at the electrode-solution interface and indeed many that are used effectively are known to be surface active. The role of coordination chemistry in the detailed mechanism of action remains somewhat obscure, not so much because the appropriate questions have not been answered, but rather because they have rarely been posed. 

Terminally metallizable dyes (30) are obtained by the interaction of a diazonium salt and a coupling component containing a chelating system, for example salicylic acid, catechol, salicyl-aldoxime or 8-hydroxyquinoline, and their coordination chemistry is typical of these compounds. Such dyes were rarely used as preformed metal complexes but were usually applied to cotton and then converted to their copper complexes on the fibre to improve their fastness to wet treatments. A typical example is the blue dyestuff (31). 

In commencing a discussion of coordination chemistry aspects of catalysis, it is well to look more closely at what is meant by the term catalyst in relation to homogeneous catalysis. In general the word catalyst is applied to that complex which is added to the reaction mixture. Our current knowledge of the mechanisms of such reactions tells us that this complex rarely, if ever, takes part in the operating catalytic cycle which converts substrate to product. This complex is therefore much better termed the catalyst precursor . 

Very few reactions of C02 complexes with nucleophiles have been reported. One of the rare examples of nucleophilic attack at C02 has been provided by Aresta et al. [81], in a study of the coordination chemistry of phenoxide ligands to Mn ... 

Bunzli, J.-C.G., 1998. Coordination chemistry of the trivalent lanthanide ions an introductory overview. In Saez Puche, R., Caro, P. (Eds.), Rare Earths. Editorial Complutense, Madrid, pp. 223-259. 

In comparison to the other 2-(2-pyridyl)-phosphole P,N-ligands 3c-f, 2,5-bis(2-pyridyl)-phosphole ligand 3b revealed a very original coordination chemistry toward Pd(I), Pt(I), and Cu(I) metal centers. A family of coordination complexes exhibiting a bridging phosphane coordination mode was thus evidenced, a very rare coordination mode for such a commonly used family of ligands. 

Rare-Earth Metals Coordination Chemistry of the Periodic Table s Footnotes ... 

The rare-earth metals are of rapidly growing importance, and their availability at quite inexpensive prices facilitates their use in chemistry and other applications. Much recent progress has been achieved in the coordination chemistry of rare-earth metals, in the use of lanthanide-based reagents or catalysts, and in the preparation and study of new materials. Some of the important properties of rare-earth metals are summarized in Table 18.1.1. In this table, tm is the atomic radius in the metallic state and rM3+ is the radius of the lanthanide(III) ion in an eight-coordinate environment. 

C. H. Huang, Coordination Chemistry of Rare-Earth Elements (in Chinese), Science Press, Beijing, 1997. 

The parameterization of a force field can be based on any type of experimental data that is directly related to the results available from molecular mechanics calculations, i. e., structures, nuclear vibrations or strain energies. Most of the force fields available, and this certainly is true for force fields used in coordination chemistry, are, at least partially, based on structural data. The Consistent Force Field (CFF)197,106,1071 is an example of a parameterization scheme where experimentally derived thermodynamic data (e. g., heats of formation) have been used to tune the force field. Such data is not readily available for large organic compounds or for coordination complexes. Also, spectroscopic data have only rarely been used for tuning of inorganic force field parameters113,74,1081. 

Structurally characterized trivalent manganese imidazole or imid-azolate complexes are also extremely rare. Current examples are limited to Mn(III) porphyrins (61), Mn(III) thiolates (Section III,B)> a Mn(III) salicylate complex (Section IV,B) and Mn(III) and Mn(IV) carboxylate complexes (Section VI,B) (62). None of these complexes contains a Mn-to-imidazole ratio greater than two consequently, no structural model for Mn SOD exists presently in Mn coordination chemistry. However, a five-coordinate Mn(II) monomer with three imidazole ligands, Mn(2-Me-ImH)3Cl2 (ImH = imidazole), has been characterized by X-ray diffraction techniques (63). 

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