Complexes and Coordination Compounds

The systematic investigation of the chemistry of the transition elements began in the nineteenth century, and it rapidly became apparent that many of the compounds were somewhat different from those with which chemists were then familiar. There was a clear difference between the behaviour of simple ionic compounds such as sodium chloride and typical transition-element compounds such as FeCl2-4H20. It was also obvious that the compounds did not resemble the typically covalent compounds of organic chemistry. It was considered that many of the compounds formed by transition metals were of a complex constitution, and they were accordingly known as complexes. 

The seminal studies on these complex compounds were conducted by Alfred Werner in an intensive period of work at the turn of the century. A typical example of the problems that Werner addressed lies in the various compounds which can be obtained containing cobalt, ammonia and chlorine. Stable and chemically distinct materials with formulations Co(NH3) Cl3 (n = 4,5 or 6) can be isolated. The concepts of valency and three-dimensional structure in carbon chemistry were being developed at that time, but it was apparent that the same rules could not apply to 

The interactions in such compounds are now better understood, and the term complex now has a more specific meaning. Not all transition-metal compounds are complexes, but many are. The terms complex and coordination compound are now used almost interchangably. 

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HOWTO NAME (/-METAL COMPLEXES AND COORDINATION COMPOUNDS... 

EXAMPLE 16.1 Sample exercise Naming complexes and coordination compounds... 

Many complexes and coordination compounds exist as isomers, compounds that contain the same numbers of the same atoms but in different arrangements. For example, the ions shown in (13a) and (13b) differ only in the positions of the Cl ligands, but they are distinct species, because they have different physical and chemical properties. Isomerism is of more than academic interest for example, anticancer drugs based on complexes of platinum are active only if they are the correct isomer. The complex needs to have a particular shape to interact with DNA molecules. 

In the last decade, an immense amount of experimental material has been generated describing the preparation and the chemical and physical properties of transition metal n complexes and coordination compounds. Recently great emphasis has been placed on the study of the kinetics and the reaction mechanisms involving such compounds. Although redistribution reactions as defined earlier in this review and as exemplified specifically by the reaction of Eq. (168) (M = transition metal, L=coordinated ligand)... 

BeMiller, J. N. (1965b). Organic complexes and coordination compounds of carbohydrates. In Starch Chemistry and Technology, Whistler. R. L., and Paschall, E. F. (Eds.), Vol. I, pp. 309-329. Academic Press, New York. 

There is no difference between donor-acceptor complexes and coordination compounds from a nomenclature point of view. Thus, for such systems an additive name such as described in Sections IR-7.1 to IR-7.3 and in Chapter IR-9 may be given. 

Metal complexes and coordination compounds are synonymous with each other, and the terms are used interchangeably for that reason. At the heart of every coordination compound is a metal atom, surrounded by electron donors called ligands. These ligands, bound directly to a metal center are considered to be within the inner coordination sphere. 

Allene complexes and coordination compounds that readily bind allenes often catalyze oligomerization and polymerization of allenes. Allene, phosphine, phosphine-carbonyl, phosphite, olefin, carbonyl, etc. complexes of metals of groups 8-10 exemplify such behavior, for instance, [Ni P(OR)3 4], [Ni(PR3)4], [Ni(CO)2 (PR3)2], [Ni(COD)2], [Rh2Cl2(CO)4], [RhCl(PPh3)3], [Rh(acac)(CH2 = C = CH2)2], etc. 

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