Structure and Isomerism in Coordination Compounds

If the sample is attracted into the field of the magnet, the left pan of the balance will be subjected to a downward force.This force is balanced by weights added to the right pan. 

Recall that the wavelength of light absorbed by a substance is related to the difference between two energy levels in the substance. See Section 7.3. 

Although we described the formation of a complex as a Lewis acid—base reaction (Section 23.3), we did not go into any details of structure. We did not look at the geometry of complex ions or inquire about the precise nature of the bonding. Three properties of complexes have proved pivotal in determining these details. 

Isomerism Isomers are compounds with the same molecular formula (or the same simplest formula, in the case of ionic compounds) but with different arrangements of atoms. Because their atoms are differently arranged, isomers have different properties. There are many possibiUties for isomerism in coordination compounds. The smdy of isomerism can lead to information about atomic arrangement in coordination compounds. Wauer s research on the isomerism of coordination compounds finally convinced others that his views woe essentially correct. 

Color A substance is colored because it absorbs fight in the visible region of the spectrum. The absorption of visible light is due to a transition between closely spaced electronic energy levels. As you have seen, many coordination compounds are highly colored. The color is related to the electronic structure of the compounds.  

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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. 

Isomers can be broadly divided into two major classes constitutional isomers and stereoisomers. In Chapter 25 we discussed isomerism in coordination compounds, and in Chapter 27 we learned about some isomeric organic compounds. In this chapter we will take a more systematic look at some three-dimensional aspects of organic structures—a subject known as stereochemistry ( spatial chemistry ). 

NOMENCLATURE AND ISOMERISM IN COORDINATION CHEMISTRY We introduce the nomenclature used for coordination compounds. We see that coordination compounds exhibit isomerism, in which two compounds have the same composition but different structures, and then look at two types structural isomers and stereoisomers. 

In this chapter, the fundamental features of the infrared spectra of inorganic compounds were introduced. Effects, such as the degree of hydration, on the appearance of infrared spectra were described. The infrared spectra of inorganic molecules are determined by the normal modes of vibration exhibited by such molecules and these were sunnnarized and common examples provided. Infrared spectroscopy is widely used to characterize coordination compounds and the important effects of coordination were introduced. Isomerism in coordination compounds may also be characterized by using infrared techniques and examples were provided. Infrared spectroscopy is extensively used to characterize metal carbonyl compounds and examples of how the bonding in such compounds may be understood were provided. The main infrared bands associated with organometaUic compounds were also introduced. Finally, examples of how infrared spectroscopy may be employed to understand the structures of mineral compounds were included. 

In this section, we will investigate the relationship between structure and isomerism. In the following sections, we will look into explanations of the paramagnetism and color of coordination compounds. 

Isomers are compounds with the same chemical composition but different structures, and the possibility of their occurrence in coordination compounds is manifest. Their importance in the early elucidation of the stereochemistries of complexes has already been referred to and, though the purposeful preparation of isomers is no longer common, the preparative chemist must still be aware of the diversity of the compounds which can be produced. The more important types of isomerism are listed below. 

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. 

Consideration of the octahedral model in accordance with symmetry knowledge also has been used to predict the presence of the mirror isomerism in complex compounds with definite content and structure. Its discovery was made by Werner in 1911, and is a confirmation of the coordination theory. ... 

We broadly divide the isomerism observed in coordination compounds into two categories structural isomers, in which atoms are connected differently to one another, and stereoisomers, in which atoms are connected in the same way but the ligands have a different spatial arrangement about the metal atom. 

Violet, easily hydrolysed, PdFp is produced when Pd [Pd F(sl s refluxed with SCF4 and is notable as one of the very few paramagnetic compounds of Pd. The paramagnetism arises from the configuration of Pd which is consequent on its octahedral coordination in the rutile-type structure (p. 961). The dichlorides of both Pd and Pt are obtained from the elements and exist in two isomeric forms which form i.s produced depends on the exact experimenial conditions used. The more usual a form of PdCb is a red material with... 

Figure 16.18 summarizes the types of isomerism found in coordination complexes. The two major classes of isomers are structural isomers, in which the atoms are connected to different partners, and stereoisomers, in which the atoms have the same partners but are arranged differently in space. Structural isomers of coordination compounds are subdivided into ionization, hydrate, linkage, and coordination isomers. 

An interesting study with BATO compounds concerns the linkage isomerization of coordinated [NCS] in [Tc(NCS)(cdoh)2(cdo)(BMe)] (440). Both a red, N-bond isothiocyanato complex [Tc(NCS)(cdoh)2(cdo)(BMe)] and the brown, S-bond thiocyanato complex [Tc(SCN)(dmg)3(BR)] (441) were isolated from the direct synthesis from [Tc04] . The N-bound (440) was found to be thermodynamically more stable than the S-bound isomer (441) and the conversion was monitored elegantly by spectroscopic methods. The X-ray crystal structure of (440) was determined. The NCS and SCN -substituted analogues exhibited r c N stretch at 2,114-2,124 cm and at 2,055-2,079 cm , respectively. " (Scheme 59). 

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