Bond Theory VBT

Pauling s valence bond theory is likewise of only limited value in its application to transition metal complexes. In the VBT, the ligand electrons are accommodated in hybrid orbitals localized on the central metal. The orbitals of interest for transition metals are the nd, n -f 1), n + l)p, and n + )d. An octahedral configuration arises from d sp hybridization of the metal orbitals, while dsp hybridization gives the square planar structure and sp hybridization results in a tetrahedral geometry. 

Thus one can make a prediction of the geometry of a metal complex based on the number and types of metal orbitals available for bonding. It is expected that the metals at the beginning of a periodic series will have a sufficient number of vacant d orbitals to give octahedral (d sp ) complexes. Also, a 

Dibenzenechromium(O) has an octahedral configuration consistent with d sp hybridization of the chromium atomic orbitals, (Fig. 3-2). If we accept this formulation of the electronic configuration of dibenzene chromium(O), this TT-complex can be classified as a coordination compound. The electronic configuration of most metal 7i-complexes thus can be accounted for by VBT. Therefore, metal r-complexes can be regarded as a new class of coordination compounds. 

However, the VBT can be considered as only a limited qualitative explanation of bonding in transition metal complexes. It cannot account for or predict the observed spectra and detailed magnetic properties of transition metal complexes nor can it predict, even qualitatively, the relative energies of different structures. It nevertheless is quite useful for predicting the geometry of complexes. 

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Soon after the development of the quantum mechanical model of the atom, physicists such as John H. van Vleck (1928) began to investigate a wave-mechanical concept of the chemical bond. The electronic theories of valency, polarity, quantum numbers, and electron distributions in atoms were described, and the valence bond approximation, which depicts covalent bonding in molecules, was built upon these principles. In 1939, Linus Pauling s Nature of the Chemical Bond offered valence bond theory (VBT) as a plausible explanation for bonding in transition metal complexes. His application of VBT to transition metal complexes was supported by Bjerrum s work on stability that suggested electrostatics alone could not account for all bonding characteristics. 

The application of organometallic compounds in medicine, pharmacy, agriculture and industry requires the accurate determination of these metals as part of their application. Most % complexes characterised by direct carbon-to-carbon metal bonding may be classified as organometallic and the nature and characteristics of the n ligands are similar to those in the coordination metal-ligand complexes. The -complex metals are the least satisfactorily described by crystal field theory (CFT) or valence bond theory (VBT). They are better treated by molecular orbital theory (MOT) and ligand field theory (LFT). There are several uses of metal 7i-complexes and metal catalysed reactions that proceed via substrate metal rc-complex intermediate. Examples of these are the polymerisation of ethylene and the hydration of olefins to form aldehydes as in the Wacker process of air oxidation of ethylene to produce acetaldehyde. 

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. 

We have noted several times in this book that resonance structures are inherently a valence bond theory (VBT) concept. Molecular orbital theory (MOT) does not require such structures. Hence, there are MOT bonding concepts that describe the bonding pictures given above for alkenes, alkynes, and CO. A simple MOT picture is given in the following Going Deeper highlight. 

The practice of electron pushing is solely a bookkeeping method—a notation. It does not represent the real movement of electrons. This means that the use of a double headed arrow to show the flow of two electrons does not literally mean that electrons are actually moving around, within and between molecules in the matter drawn. Nevertheless, the notation is useful within a valence bond theory (VBT) context, because it indicates how discrete bonds and lone pairs have been rearranged when comparing the reactant to the product. 

There are two methods for finding such approximate solutions, namely the Valence Bond Theory (VBT) and the Molecular Orbital Theory (MO-theory). 

From the point of view of the Valence Bond Theory (VBT) the conception of the chemical bonding is practically the same as that of R.N. Lewis. VBT may indeed be considered as representing a rationalization in terms of the wave mechanics of the idea of bonds by sharing electron pairs. 

The bonding in transition metal complexes has been described by three different theories crystal field theory (CFT), valence bond theory (VBT), and molecular orbital theory (MOT). Detailed descriptions of these three approaches are given in the standard inorganic texts and are not repeated here. However, some general statements concerning the applicability of these various bonding descriptions for metal 7r-complexes are noted. 

The introduction of the term electronegativity (%) to chemical literature is credited to Professor Linus Pauling, a pioneer towards the development of the valence bond theory (VBT) [19, 20]. Pauling, in an attempt to explain the factor of extra stabilization of a heteronuclear chemical bond A - B (between two different atoms A and B) obtained from its allied homonuclear analogues (A - A and B - B) during a chemical combination process, proposed that, there actually occurs a difference in the aptitude of electron donation (or acceptance) between the interacting atoms. He defined this behavior as the ability of an atom (or a functional group) in a molecule to attract bonded electrons (or electron density) towards itself. 

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