Bond, covalent molecular orbital description

A covalent bonding model is needed to account for the effects of Jt-donor or 7i-acceptor ligands and synergic bonding. A molecular orbital description provides a more universal description of bonding within transtion metal complexes. 

The electronic state calculations of transition metal clusters have been carried out to study the basic electronic properties of these elements by the use of DV-Xa molecular orbital method. It is found that the covalent bonding between neighboring atoms, namely the short range chemical interaction is very important to determine the valence band structure of transition element. The spin polarization in the transition metal cluster has been investigated and the mechanism of the magnetic interaction between the atomic spins has been interpreted by means of the spin polarized molecular orbital description. For heavy elements like 5d transition metals, the relativistic effects are found to be very important even in the valence electronic state. 

We said in Section 1.6 that chemists use two models for describing covalent bonds valence bond theory and molecular orbital theory. Having now seen a valence bond description of the double bond in ethylene, let s also look at a molecular orbital description. 

VSEPR concept, valence bond description and hybridization, molecular orbital description, bond energies, covalent and van der Waals radii of the elements, intermolecular forces... 

Localized and Delocalized Molecular Orbital Description of Covalent Bonds... 

That is, the first excited state of the molecular orbital description is identical to the first excited state in the valence bond description. Both describe a covalent repulsive state that separates to two free H atoms. The molecular orbital description of the other excited state is ... 

This element crystallizes with the diamond structure involving covalent bonds formed from sp hybrid silicon atoms. A molecular orbital description of bonding based on the sp hybrid orbitals shows that the gap between... 

Molecular orbital (MO) theory (Section 1.11) A description of covalent bond formation as resulting from a mathematical combination of atomic orbitals (wave functions) to form molecular orbitals. 

In other words, the valence bonds approach is suitable for compounds showing purely ionic or purely covalent behaviour we require molecular orbitals for a more mature description of the bonding in such materials. So the yellow colour of silver iodide reflects the way the bonding is neither ionic nor covalent. We find, in fact, that the charge clouds of the silver and iodide ions overlap to some extent, allowing change to transfer between them. We will look at charge transfer in more detail on p. 459. 

A second quantum mechanical bonding theory is molecular orbital theory. This theory is based on a wave description of electrons. The molecular orbital theory assumes that electrons are not associated with an individual atom but are associated with the entire molecule. Delocalized molecular electrons are not shared by two atoms as in the traditional covalent bond. For the hydrogen molecule, the molecular orbitals are formed by the addition of wave functions for each Is electron in each hydrogen atom. The addition leads to a bonding molecular... 

The valence bond model of covalent bonding is easy to visualize and leads to a satisfactory description for most molecules. It does, however, have some problems. Perhaps the most serious flaw in the valence bond model is that it sometimes leads to an incorrect electronic description. For this reason, another bonding description called molecular orbital (MO) theory is often used. The molecular orbital model is more complex than the valence bond model, particularly for larger molecules, but sometimes gives a more satisfactory accounting of chemical and physical properties. 

The covalent description has appeal from the standpoint of explaining some behaviors not readily handled by the electrostatic model. On the other hand, it leaves the difficulty of fitting extravalency into our present valence bond understanding of the covalent bond. This difficulty emphasizes the importance of clarifying the theory of the H bond, and it may foreshadow chemges in the theory of the chemical bond, possibly including a turn toward the molecular orbital approach. 

How does electron sharing occur Two models have been developed to describe covalent bond formation valence bond theory and molecular orbital theory. Each model has its strengths and weaknesses, and chemists use them interchangeably depending on the circumstances. Valence bond theory is the more easily visualized of the two, so most of the descriptions we ll be using in this book derive from that approach. We ll take a brief look now at both theories and then return for a second look at molecular orbital theory in Section 1.9. 

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