Bonding theories Werner

The nature of the noncovalent bonds that link molecules into supramolecular clusters has inspired discussion and speculation throughout the history of valence theory. Werner s transition-metal studies originally led to the concept of near-valence ... 

As new complex ions were synthesized, several bonding theories were postulated and rejected. The two most convincing theories, the Blomstrand-Jorgensen chain theory and coordination theory proposed by Alfred Werner, were debated extensively (a subject taken up in detail in Chapter 2) and it was coordination theory that eventually proved to be correct, winning Wemer the Nobel Prize in 1913. 

Werner s coordination theory, with its concept of secondary valence, provides an adequate explanation for the existence of such complexes as [Co(NH3)6]Cl3-Some properties and the stereochemistry of these complexes are also explained by the theory, which remains the real foundation of coordination chemistry. Since Werner s work predated by about twenty years our present electronic concept of the atom, his theory does not describe in modem terms the nature of the secondary valence or, as it is now called, the coordinate bond. Three theories currently used to describe the nature of bonding in metal complexes are (1) valence bond theory (VBT), (2) crystal field theory (CFT), and (3) molecular orbital theory (MOT). We shall first describe the contributions of G. N. Lewis and N. V. Sidgwick to the theory of chemical bonding. 

One night in late 1892, at the age of 26, Alfred Werner was awakened from his sleep at two in the morning. He had just had a dream about the nature of bonding in coordination compounds. For the next 15 h, he recorded his insights in what eventually became his third scientific paper. Unlike the chain theory, Werner postulated that all of the cobalt compounds in Table 15.1 had two types of valences primary (Hauptvalenz) and secondary valence. Furthermore, every metal had a fixed coordination number equal to the number of ligands that were directly bonded to... 

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. His theory of coordination chemistry was published in 1893 when Werner was 26 years old. In his paper Werner made the revolutionary suggestion that metal ions such as Co3+ could show two different kinds of valences. For the compound Co(NH3)eCI3, Werner postulated a central Co3+ ion joined by "primary valences" (ionic bonds) to three Cl- ions and by "secondary valences"... 

The main difference between the two theories lies in Werner s assumption that the co-ordinated groups surrounding the central atom are connected with that atom by valency bonds, whereas Friend assumes them to be connected with one another. The accepted method of writing the formula with the central groups within square brackets remains the same in both cases. 

The fact that selective binding must involve attraction or mutual affinity between host and guest. This is, in effect, a generalisation of Alfred Werner s 1893 theory of coordination chemistry, in which metal ions are coordinated by a regular polyhedron of ligands binding by dative bonds. 

Considerable progress in the development of theoretical and synthetic coordination and organometallic chemistry was made with the use of electron ideas. Lewis elaborated in 1923 the classic electron theory of acids and bases [30], and used it to explain the coordination ideas of Werner [31] (in Ref. 32, this achievement is ascribed to Sidgwick). A Lewis acid (A) is a acceptor of the electron pair and a Lewis base (B) is its donor [33], In other words, A is a species that can form a new covalent bond by accepting a pair of electrons and B is a species that can form a new covalent bond by donating a pair of electrons. The fundamental Lewis acid-base theory is described by a direct equlibrium [Scheme (1.1)], leading to the formation of the adduct (acid-base complex) ... 

The classical rules of valency do not apply for complex ions. To explain the particularities of chemical bonding in complex ions, various theories have been developed. As early as 1893, A. Werner suggested that, apart from normal valencies, elements possess secondary valencies which are used when complex ions are formed. He attributed directions to these secondary valencies, and thereby could explain the existence of stereoisomers, which were prepared in great numbers at that time. Later G. N. Lewis (1916), when describing his theory of chemical bonds based on the formation of electron pairs, explained the formation of complexes by the donation of a whole electron pair by an atom of the ligand to the central atom. This so-called dative bond is sometimes denoted by an arrow, showing the direction of donation of electrons. In the structural formula of the tetramminecuprate(II) ion... 

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