Other methods of studying coordination compounds

In preceding chapters the theories underlying some of the methods which have been used to characterize coordination compounds have been discussed in some detail. The present chapter provides a less detailed survey of other methods, stretching from indicators of complex formation through to some which provide insights into physical structure, others into electronic structure, yet others into the forces between atoms and molecules. 

If the second equilibrium exists, that is, if a complex is formed, addition of more A will cause the first equilibrium to be displaced to the left and the pH of the solution will decrease. Measurement of pH not only allows the formation of complexes to be detected but may also be used to determine the stability constants of the species formed (see Chapter 5). As will be seen, half-cell potentials and polarographic measurements may be used both to detect complex formation and to determine stability constants but, before turning to these a variety of techniques will be outlined which not only indicate the formation of coordination complexes but also provide information on their structure. 

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Trialkyl- and triarylarsine sulfides have been prepared by several different methods. The reaction of sulfur with a tertiary arsine, with or without a solvent, gives the sulfides in almost quantitative yields. Another method involves the reaction of hydrogen sulfide with a tertiary arsine oxide, hydroxyhahde, or dihaloarsorane. X-ray diffraction studies of triphenylarsine sulfide [3937-40-4], C gH AsS, show the arsenic to be tetrahedral the arsenic—sulfur bond is a tme double bond (137). Triphenylarsine sulfide and trimethylarsine sulfide [38859-90-4], C H AsS, form a number of coordination compounds with salts of transition elements (138,139). Both trialkyl- and triarylarsine selenides have been reported. The trialkyl compounds have been prepared by refluxing trialkylarsines with selenium powder (140). The preparation of triphenylarsine selenide [65374-39-2], C gH AsSe, from dichlorotriphenylarsorane and hydrogen selenide has been reported (141), but other workers could not dupHcate this work (140). 

Apart from the determination of the structures of stannylenes by diffraction methods (X-ray or electron diffraction) many other physico-chemical techniques can be exployed to characterize these compounds more completely. Besides the classical methods such as IR-, Raman-, PE-, UV- and NMR-spectroscopy, MoBbauer-119 m-tin spectroscopy is widely used for the determination of the oxidation states of tin atoms and of their coordination 1n8-12°-123>. jt is not in the scope of this report to study the dependence of MoBbauer constants such as isomer shift and quadrupole splitting on structural parameters. Instead, we want to concentrate on one question Which information can we deduce from the structure of stannylenes to evaluate their reactivity ... 

The great recent development in electrochemical techniques will certainly be helpful for the study of redox processes of a metal which can occur in so many oxidation states. Multinuclear NMR spectrometers will allow increased use of 51V resonance as a routine method for the characterization of complexes in solution. Other recent developments are the study of polynuclear complexes, metal clusters (homo and hetero-nuclear) and mixed valence complexes, and it can be anticipated that these topics will soon become important areas of vanadium coordination chemistry, although the isolation of compounds with such complex... 

Prussian blue — iron(III) hexacyanoferrate(II) is the archetype of sparingly soluble mixed valence polymeric metal hexacyanometalates with the formula Me Me(N) [Me c (CN)6] with (i), (N), and (C) indicating the position in the crystal lattice, where (i) means interstitial sites, (N) means metal coordinated to the nitrogen of the cyanides, and (C) means metal ions coordinated to the carbon of the cyanides. It is one of the oldest synthetically produced coordination compounds and was widely used as pigment in paints because of the intensive blue color. The compound has been studied extensively by electrochemical and other methods. The importance of Prussian blue in electrochemistry is related to the fact that it has two redox-active metal centers and that it has an open structure that allows small cations to... 

In the early 19th century, Gay-Lussac discovered F Si 2 NHj. Three years later Davy obtained the same compound and Davy and Berzelius synthesized other compounds of six-coordinate silicon fluorosilicic acid, HjPiFg], and its salts, the hexafluorosilicates In the second half of the last century adducts of silicon tetrahalides with tertiary amines were synthesized . In 1903 Dilthey investigated silicon diketonates containing six-coordinate silicon In the first half of our century the possibility of the presence of Si(OH)g ions in silica was widely discussed Interest in five- and six-coordinate silicon compounds increased in the second half of the present century. As a result, new classes of silicon compounds with an expanded coordination sphere were discovered and were studied by various physical and chemical methods. Specific biological activity was observed for some of the compounds and promising ways of their practical application were planned... 

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