Ligands organometallic chemistry

Gadolinium ions are highly toxic in their hydrated form [Gd(H20)g] +. In vivo, Gd + ions compete with calcium ions and block the reticuloendothelial system. To avoid this toxicity, gadolinium is complexed with an organic thermodynamically stable matrix, (see Lanthanide Complexes with Multidentate Ligands, Organometallic Chemistry Fundamental Properties, Lanthanides Coordination Chemistry) In addition, ligands that allow coordination sites for one (or more) water molecule(s) ate preferred since they increase... 

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected stnicture types have been synthesized in which polyhedral boranes or their anions can be considered to act as ligands which donate electron density to metal centres, thereby forming novel metallaboranc elusters, ". Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helpful m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride clu.ster chemistry, metallaboranc and metallacarbaborane chemistry (pp. 189-95). organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

Also in the divalent state, Pd and Pt show the class-b characteristic of preferring CN and ligands with nitrogen or heavy donor atoms rather than oxygen or fluorine. Platinum(IV) by contrast is more nearly class-a in character and is frequently reduced to Pt by P- and Aj-donor ligands. The organometallic chemistry of these metals is rich and varied and that involving unsaturated hydrocarbons is the most familiar of its type. 

Volume 78 of Advances in Heterocyclic Chemistry contains four contributions. A. P. Sadimenko of Fort Hare University of South Africa has covered organometal-lic compounds of furan, thiophene, and their benzannulated derivatives. This constitutes the first installment of a projected series on the organometallic chemistry of heteroaromatic ligands, a subject of great fundamental and technical importance that has exploded in the 1990s. 

Generalizaticm of the organometallic chemistry of heterocycles is an important task It allows us to understand the basic trends of the ligand behavior of heterocycles, their coordination modes, and dieir application in various branches of applied chemistry, especially materials chemistry [93CCR(126)237 96CCR(147)247 01AHC(78)1 01AHC(79)115]. 

Some ligands by being analogues of azoles (condensed diphospholyls, SeP, TeP, SS, and S2N3) are occasionally examined but no systematic study of their organometallic chemistry exists. 

R. Davis, L. A. P. Kane-Maguire in Molybdenum Compounds with r2 - >/fJ Ligands in Comprehensive Organometallic Chemistry, G. Wilkinson, F. G. A. Stone, W. Abel, Eds., Vol. 3, p 1149fF., Pergamon, Oxford 1982. 

Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis. C. A. Tolman, Chem. Rev., 1977, 77, 313-348 (298). 

The versatility of tertiary phosphane ligands in coordination and organometallic chemistry. R. Mason and D. W. Meek, Angew. Chem., Int. Ed. Engl., 1978,17,183-194 (138). 

Development of more efficient transition metal catalyst systems including using novel and efficient ligands has been one of the focuses in organometallic chemistry.35 The developments in this area will allow not only to synthesize polymers under mild conditions with higher or desired molecular weights but also to use less expensive, more readily available materials for the polymerizations. 

Low oxidation states - An important characteristic of transition metal chemistry is the formation of compounds with low (often zero or negative) oxidation states. This has little parallel outside the transition elements. Such complexes are frequently associated with ligands like carbon monoxide or alkenes. Compounds analogous to Fe(CO)s, [Ni(cod)2] (cod = 1,4-cyclooctadiene) or [Pt(PPh3]3] are very rarely encountered outside the transition-metal block. The study of the low oxidation compounds is included within organometallic chemistry. We comment about the nature of the bonding in such compounds in Chapter 6. 

Various phosphines are valuable ligands for metal centers involved in catalytic and noncatalytic organometallic chemistry. Unfortunately, phosphines are readily oxidized to phosphine oxides and to prevent their oxidation they... 

Bis(cyclopentadienyl) complexes are central to the organometallic chemistry of the early transition metals and feature in applications such as alkene polymerization chemistry. Parallels can be drawn between a porphyrin ligand and two cyclopentadienyl ligands, in that they both contribute a 2— formal charge and exert a considerable steric influence on other ligands in the same molecule. Several of the metalloporphyrin complexes discussed below have bis(cyclopentadienyl) counterparts, and authors in some ca.ses have drawn quite detailed comparisons, although these discussions will not be repeated here. 

The field of transition metal complexes of isocyanides developed slowly over more than a century to a respectable subarea in coordination chemistry, and in the process seems to have attracted very little attention. Even the remarkable resurgence of transition metal organometallic chemistry in the last 20 years, and the realization that isocyanides and carbon monoxide should be quite similar as ligand groups in organometallic complexes, did not initiate an extensive development of this area of chemistry. Only in the last several years has this potentially important subject begun to receive the attention it would seem to deserve. 

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