Theories of coordination compounds

Solvates are perhaps less prevalent in compounds prepared from liquid ammonia solutions than are hydrates precipitated from aqueous systems, but large numbers of ammines are known, and their study formed the basis of Werner s theory of coordination compounds (1891-5). Frequently, however, solvolysis (ammonolysis) occurs (cf. hydrolysis). Examples are ... 

We saw in Section 1.3 how Alfred Werner formulated the modern concept of coordination chemistry, which supramolecular chemistry generalises to a complete coordination chemistry . Prior to Werner s time the chain theory of coordination compounds was popular. The chain and Werner formulations of Co (NIT3) 4CI3 and Co (NH3) 3C13 are shown in Figure 3.1. While both theories predict that Co (NH3) 4C13 will exhibit one labile chloride ion per molecule, the chain theory also predicts that Co(NH3)3C13 will have one labile chloride, while Werner s theory ultimately correctly predicted that the chloride is not labile in this case. 

Symmetry has a major role in two widely used and successful approaches of chemistry, viz., the crystal field and ligand field theories of coordination compounds. This topic has been thoroughly covered in textbooks and monographs on coordination chemistry. Therefore, it is mentioned here only in passing. 

Historically Important Coordination Compounds Some Pre-Wemer Theories of Coordination Compounds... 

Because sufficient experimental data must be accumulated before attempts can be made to explain them and to predict new phenomena, theories usually lag behind practice. During the first half of the nineteenth century, discoveries of coordination compounds were few, sporadic, and often accidental. Only after Gibbs and Genth s work did chemists devote themselves to systematic studies of these compounds. It might therefore be expected that few theories of coordination compounds would be proposed until late in the second half of the nineteenth century. In fact, theories were devised promptly because of the great importance of coordination compounds to the general problem of chemical bonding. 

The Werner theory of coordination compounds was based on a group of compounds that is relatively slow to react in solution and thus easier to study. For this reason, many of his examples were compounds of Co(III), Rh(III), Cr(III), Pt(II), and Pt(IV), which are kinetically inert or slow to react Examination of more reactive compounds over the years has confirmed their similarity to those originally smdied, so we will include examples of both types of compounds in the descriptions that follow. 

Walter Hleber (1895-1976) was a student of Rudolf Weinland, who performed early experimental work on Alfred Werner s theory of coordination compounds (Hauptvalenzen, Nebenvalenzen). Hieber received his Ph. D. in 1919 from Tubingen University on a topic concerning ferric complexes of hypophosphorous acid. He then developed metal carbonyl chemistry, mainly at Technische Hochschule Miinchen (1935-1964) he is now considered the pioneering researcher in this area of study. His name is associated with compounds like HCo(CO)4 and H2pe(CO)4 that are relevant to catalytic hydrogen-transfer reactions (hydroformylation Section 2.1.1). Nucleophilic addition to metal carbonyls, e. g., Fe(CO)5 -i-OH —> [(C0)4FeC(=0)0H] , is known as the Hieber base reaction (cf. [76]). 

Perry, R. Keller, R. Modem Developements - The Electrostatic Theory of Coordination Compounds. New York 1956... 

Werner s most important contribution, his theory of valency and the structure of coordination compounds, was first presented in 1891 to qualify for a post in the Zurich Polytechnic. He assumed that the valency of an atom, including the carbon atom, is an attractive force emanating from the centre and acting uniformly towards all parts of the surface, rather than directed valency bonds. Although he claimed that this would lead to van t Hoff s configurational formulae, it is on the basis of the latter and directed bonds that Werner s own theory of coordination compounds has been most successfully explained. 

Alfred Werner (1866-1919). Swiss chemist. Werner started as an organic chemist but became interested in coordination chemistry. For his theory of coordination compounds, Werner was awarded the Nobel Prize in Chemistry in 1913. 

But Kekuld s stability criterion, or to be more accurate, instability criterion failed completely in the case of many coordination compounds, especially the metal-ammines, which were classified as molecular compounds by sheer dint of necessity even though they were extremely resistant to heat and chemical reagents. For example, look at Figure 1. Although hexaamminecobalt(III) chloride contains ammonia, it neither evolves this ammonia on mild heating nor does it react with acids to form ammonium salts. Also, despite its cobalt content, addition of a base to its aqueous solution fails to precipitate hydrated cobalt(III) hydroxide. It remained for Alfred Werner to explain successfully the constitution of such compounds, but the time was not yet ripe. Before considering Werner s coordination theory, we must examine one more theory of coordination compounds, perhaps the most successful of the pre-Wemer theories, namely, the Blomstrand-J0rgensen chain theory. 

The most successful theory of coordination compounds prior to that of Alfred Werner was advanced in 1869 by Christian Wilhelm Blomstrand (1826-1897) (Figure 12.4). In order to account for the lack of reactivity of ammonia in the ammines, he proposed that chains of ammonia molecules linked the metal atom to other parts of the molecule (Figure 12.5). Blomstrand thereby assigned nitrogen a... 

Cyanide complexes are important to the theory of coordination compounds because of their type of bonding and their exceptional stability. 

Werner literally dreamed up the modern theory of coordination compounds in 1892. He envisioned that metals had two types of valence, which we refer to today as oxidation state and coordination number. Some ligands satisfy only the coordination number, whereas others simultaneously satisfy the oxidation state. These ideas explain why some chlorides in the cobalt ammonate chlorides are ionizable and some are not. By comparing the actual number of known isomers with the number that should exist for various geometries, Werner concluded that the six ligands in the cobalt ammonates were in an octahedral arrangement. 

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