Structure and Bonding in Coordination Compounds

The chemistry of coordination compounds is a broad area of inorganic chemistry that has as its central theme the formation of coordinate bonds. A coordinate bond is one in which both of the electrons used to form the bond come from one of the atoms, rather than each atom contributing an electron to the bonding pair, particularly between metal atoms or ions and electron pair donors. Electron pair donation and acceptance result in the formation of a coordinate bond according to the Lewis acid-base theory (see Chapter 5). However, compounds such as H3N BC13 will not be considered as coordination compounds, even though a coordinate bond is present. The term molecular compound or adduct is appropriately used to describe these complexes that are formed by interaction of molecular Lewis acids and bases. The generally accepted use of the term coordination compound or coordination complex refers to the assembly that results when a metal ion or atom accepts pairs of electrons from a certain number of molecules or ions. Such assemblies commonly involve a transition metal, but there is no reason to restrict the term in that way because nontransition metals (Al3+, Be2+, etc.) also form coordination compounds. 

Numerous types of important coordination compounds (such as heme and chlorophyll) occur in nature. Some coordination compounds are useful as a result of their ability to function as catalysts for industrially important processes. Also, the formation of coordination compounds is central to certain techniques in analytical chemistry. Accordingly, some understanding of the chemistry of coordination compounds is vital to students whose interests lie outside inorganic chemistry. Certainly, the field of coordination chemistry is much broader in its applicability than to just inorganic chemistry. 

In Chapter 5, Lewis bases (electron pair donors) were classified as nucleophiles and electron pair acceptors were designated as electrophiles or Lewis acids. These concepts will now be used to describe coordination complexes of metals. 

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In previous chapters, we have presented a great deal of information about structure and bonding in coordination compounds. This chapter will be devoted to describing some of the important chemistry in the broad areas of organometallic complexes and those in which there are metal-metal bonds. The body of literature on each of these topics is enormous, so the coverage here will include basic concepts and a general survey. 

House, James E., and House, Kathleen A. (2001). Structure and Bonding in Coordination Compounds (Chapter 19) and Synthesis and Reactions of Coordination Compounds (Chapter 20). In Descriptive Inorganic Chemistry. San Diego Harcourt/Academic Press. 

The subjects of structure and bonding in metal isocyanide complexes have been discussed before 90, 156) and will not be treated extensively here. A brief discussion of this subject is presented in Section II of course, special emphasis is given to the more recent information which has appeared. Several areas of current study in the field of transition metal-isocyanide complexes have become particularly important and are discussed in this review in Section III. These include the additions of protonic compounds to coordinated isocyanides, probably the subject most actively being studied at this time insertion reactions into metal-carbon bonded species nucleophilic reactions with metal isocyanide complexes and the metal-catalyzed a-addition reactions. Concurrent with these new developments, there has been a general expansion of descriptive chemistry of isocyanide-metal complexes, and further study of the physical properties of selected species. These developments are summarized in Section IV. 

Although the simple valence-bond approach to the bonding in coordination compounds has many deficiencies, it is still useful as a first attempt to explain the structure of many complexes. The reasons why certain ligands force electron pairing will be explored in Chapter 17, but it is clear that high- and low-spin complexes have different magnetic character, and the interpretation of the results of this technique will now be explored. 

DeKock, R. L., and Gray, H. B. (1980). Chemical Structure and Bonding. Benjamin/Cummings, Menlo Park, CA. Chapter 6 presents a good introduction to bonding in coordination compounds. 

The investigation of the physical properties of inorganic substances. This may be undertaken with a view to exploiting practical applications, or to obtain chemically-relevant information from the physical measurements. Magnetic measurements, for example, may tell us something about the electronic structure and bonding in a coordination compound. 

Compounds having the same numbers and types of atoms but different structures are called isomers. Coordination compounds exhibit several of types of isomerism, and the study of these various types of isomers constitutes one of the interesting and active areas of research in coordination chemistry. Because so much of coordination chemistry is concerned with isomeric compounds, it is essential that a clear understanding of the various types of isomerism be achieved before a detailed study of structure and bonding in complexes is undertaken. Although the possibility of a substantial number of types of isomerism exists, only the more important types will be discussed here. 

Most complex ions containing Co, Cr, and Pt are kineticaUy inert. Because they exchange ligands very slowly, they are easy to smdy in solution. As a result, our knowledge of the bonding, structure, and isomerism of coordination compounds has come largely from smdies of these compounds. 

Heretofore the majority of investigators considered the chemical bond in coordination compounds of the rare earths elements to be of ionic nature. Within the last years, however, many arguments have been submitted in favour of a partial covalency of the formed bonds. One of the reasons points out at the impossibility to deduce thermodynamical characteristics of the complexes on the mere basis of the ions charges and radii. An ample information on the geometry and chemical structure of the rare earth coordination compounds may be obtained on the basis of their absorption spectra. 

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