Role in coordination chemistry

P12 This functional group has roles in coordination chemistry as a ligand, in medicinal chemistry as a metabolically stable surrogate... 

It would sound funny to say, in the past tense, that the functional group had roles in coordination chemistry, implying that those roles are no longer important. In addition to present tense, another verb construction is commonly used when citing others works. Consider the following sentence ... 

As shown above, these compounds are radical species and play an important role in coordination chemistry [125a, 169,202,203]. In nonaqueous solvents (hexane, dimethoxyethane, toluene, THF, and chloroform), mainly the formation of complexes of types 952955 (R = 3,5- or 3,6-t-Bu2) has been observed by EPR [14] ... 

Porphyrins and related tetrapyrrole macrocyclic ligands are one of the most prominent ligand types in coordination chemistry. Their occurrence in metallo-enzymes, their application as inorganic dyes and photosensitisers, their use as homogeneous catalysts, mainly for oxidation or oxygenation reactions, and their outstanding role in coordination chemistry is in general well documented. ... 

Schiff bases play an important role in coordination chemistry as they readily form stable complexes with most of the transition metals. The development of bioinorganic chemistry has increased interest in Schiff base complexes as it has been recognised that many of these complexes may serve as models for the metal-containing sites in metallo-proteins and -enzymes. Schiff also discovered the experimental technique of preparing... 

In fact, Werner played such a central and almost monopolistic role in coordination chemistry that his name is virtually synonymous with the field. Even today, almost 75 years after his death in 1919, coordination compounds, particularly metal-ammines, are still colloquially called Werner complexes. The coordination theory not only provided a logical explanation for known "molecular compounds, but also predicted series of unknown compounds, whose eventual discovery lent further weight to Werner s controversial ideas. He showed how ammonia could be replaced by water or other groups, and he demonstrated the existence of transition series between ammines, double salts, and hydrates. Werner recognized and named many types of inorganic isomerism such as coordination isomerism, polymerization isomerism, ionization isomerism, hydrate isomerism, salt isomerism, coordination position isomerism, and valence isomerism. He also postulated explanations for polynuclear complexes, hydrated metal ions, hydrolysis, and acids and bases. His view of the two types of chemical... 

Bidentate phosphine ligands play a major role in coordination chemistry and its catalytic application. Ethylene and ort/to-phenylene bridges are favored as C2-symmetric backbone. This allows the formation of stable flve-membered rings by coordination to a transition metal. It has been shown that using a phosphorus donor atom is the most viable option, owing to its lone pair of electrons, which promotes formation of o bonds, and the availability of empty o orbitals, which facilitate n back-bonding [11]. With these orbitals the phosphorns atom can be used as the Ji-acceptor for com-plexation to late transition metals. With the nse of electron-withdrawing substituents the o-donor... 

Low-coordinate species of the main group elements of the second row such as carbenes, olefins, carbonyl compounds (ketones, aldehydes, esters, amides, etc.), aromatic compounds, and azo compounds play very important roles in organic chemistry. Although extensive studies have been devoted to these species not only from the physical organic point of view but also from the standpoints of synthetic chemistry and materials science, the heavier element homologues of these low-coordinate species have been postulated in many reactions only as reactive intermediates, and their chemistry has been undeveloped most probably due to... 

The other area of coordination chemistry to be described in this chapter is concerned with the role of coordination chemistry in some biochemical processes. There are a great number of such processes that depend on the presence of some metal ion for their effectiveness. The area of bioinorganic chemistry has grown enormously in recent years, and because a metal ion is involved, the topic is considered 779... 

We illustrate the nomenclature introduced above in an example taken from coordination chemistry. In fact, equilibrium species of interesting complexity are commonly encountered in coordination chemistry and to a large extent coordination chemists have developed the principles of equilibrium studies. Consider the interaction of a metal ion M (e.g. Cu2+) with a bidentate ligand L (e.g. ethylenediamine, en) in aqueous solution. For work in aqueous solution the pH also plays an important role and thus, the proton concentration H (=[ff+]), as well as several differently protonated species, need to be taken into account. Using the nomenclature commonly employed in coordination chemistry, there are three components, M, L, and H. In aqueous solution they interact to form the following species, HL, H2L, ML, Mia, ML3, MLH, MLH1 and OH. (In fact, more species are formed, e.g. ML2H 1, but the above selection will suffice now.) The water molecules are usually not defined as additional components. The concentration of water is constant and its value is taken into the equilibrium constants. 

Another common theme that authors use to establish importance involves environmental impacts. For example, an environmental slant is used in the first sentence of the cyclodextrin article (P3, exercise 6.7), where the study of cyclo-dextrins is justified based on their role in soil remediation. The importance of work that benefits air or water quality and/or promotes green chemistry can also be stressed. Work is also viewed as important if it has cross-disciplinary applications. For example, in the Introduction section of the tetrazole article, the authors stress the importance of tetrazoles in coordination chemistry, medicinal chemistry, and in various materials science applications and point out their role as useful intermediates in the preparation of substituted tetrazoles ... 

While coordination chemistry of hydroxamic acids has been studied extensively (particularly with Cu(II), Zn(n), Fe(III), Co(III), and Cr(III) ions) there have been only few reports in the literature on the formation of Cr(V) complexes of these ligands. The current interest in coordination chemistry of Cr(V) arises mainly from the proposed crucial role of reactive Cr(V) intermediates in Cr(VI)-induced genotoxicity and carcinogenicity . ... 

Ligands having a lone electron pair on the phosphorus atom play a key role in transition-metal coordination chemistry of today. The range of complexes that can be produced by phosphorus donors is unparalleled in coordination chemistry. They are ca-... 

Carboxylates bind readily to transition metal cations (42). They are common in coordination chemistry and play a significant role at the active sites of many metalloproteins (43). 

Phosphorus trifluoride is a ligand that is used extensively in coordination chemistry. It substitutes readily into various metal carbonyl complexes using either thermal or photochemical techniques. As a ligand, it is unique in its similarity to carbon monoxide in lower-valent organometallic compounds. In its role as a model for CO, a number of studies are possible that cannot be done on the carbonyls themselves.1 The name normally used for PF3 in complexes is trifluorophosphine. 

It is now possible to indicate the role of coordination chemistry by means of a single example—the problem of determining calcium in sea water (4, 6). 

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. 

A final example concerns the formation of heteropolynuclear hydroxide complexes.116 The complexes [(OH)Fe(OH)2Cr]3+, [(OH)Fe(OH)2Cr(OH)]2+ and [(OH)2Fe(OH)2Cr(OH)]+, or polymers such as Fe(OH)2M "+ (M = V, Cr, Mn, Co, Ni, Cu n = 2-4) have been studied with a view to an understanding of the inclusion of transition metals in iron ores as mixed oxides rather than their occurrence as discrete mineral phases. Many other examples might have been chosen in this section. Reference should be made to the general reviews given above. However it should be clear that simple inorganic coordination complexes play a major role in the chemistry of natural aqueous systems at low temperatures. 

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