Molecular orbital theory ionic bond

Before leaving this brief introduction to molecular orbital theory, it is worth stressing one point. This model constructs a series of new molecular orbitals by the combination of metal and ligand orbitals, and it is fundamental to the scheme that the ligand energy levels and bonding are, and must be, altered upon co-ordination. Whilst the crystal field model probably over-emphasises the ionic contribution to the metal-ligand interaction, the molecular orbital models probably over-emphasise the covalent nature. 

In this chapter, we discuss mostly the bonding in mononuclear homoleptic complexes ML using two simple models. The first, called crystal field theory (CFT), assumes that the bonding is ionic i.e., it treats the interaction between the metal ion (or atom) and ligands to be purely electrostatic. In contrast, the second model, namely the molecular orbital theory, assumes the bonding to be covalent. A comparison between these models will be made. 

The topic of interactions between Lewis acids and bases could benefit from systematic ab initio quantum chemical calculations of gas phase (two molecule) studies, for which there is a substantial body of experimental data available for comparison. Similar computations could be carried out in the presence of a dielectric medium. In addition, assemblages of molecules, for example a test acid in the presence of many solvent molecules, could be carried out with semiempirical quantum mechanics using, for example, a commercial package. This type of neutral molecule interaction study could then be enlarged in scope to determine the effects of ion-molecule interactions by way of quantum mechanical computations in a dielectric medium in solutions of low ionic strength. This approach could bring considerable order and a more convincing picture of Lewis acid base theory than the mixed spectroscopic (molecular) parameters in interactive media and the purely macroscopic (thermodynamic and kinetic) parameters in different and varied media or perturbation theory applied to the semiempirical molecular orbital or valence bond approach [11 and references therein]. 

The second alleriiativc is the ihiec-ccnter. four-electron bond developed by simple molecular orbital theory for the noble gas lluoridcs (see Chapter 17). Since this predicts that each bonding pair of electrons (each "bond") is spread over three nuclei, the bond between two of the nuclei is less than that of a normal two-center, two-electron bond. Furthermore, since the nonbonding pair of electrons is localized on the tiuorine titoms. there is a separation of charge ("ionic character"). In both respects, then, this interpretation agrees with Pauling s approach and with the experimental facts. 

In ionic species like NaCl, the energy that holds the lattice together is the electrostatic attraction of the Na cations and the CP anions. In covalent molecules with electron pairs or stick bonds, it is less obvious what holds the molecule together. To appreciate this it is necessary to look at the Molecular Orbital Theory (MO) of simple diatomic molecules. Table 4.8 shows some experimental data for some simple diatomic molecules. What is immediately noticeable about these data is that although there is a linear increase in the total number of valence shell electrons along this series of diatomic species, neither their interatomic distances nor their heats of formation increase linearly. In fact, the shortest distance and the highest heat of formation occur for the... 

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