Bond directionality model

The X-ray crystal structures of many of the.se complexes have now been determined representative examples are. shown in Fig. 4.11 from which it is clear that, at least for the larger cations, coordinative saturation and bond directionality are far less significant factors than in many transition element complexes. - Further interest in these ligands stems from their use in biochemical modelling since they sometimes mimic the behaviour of naturally occurring, neutral, macrocyclic antibiotics such as valinomycin, monactin, nonactin, nigericin... 

The model of sphere packing requires no definition as to the nature of the bonding holding the spheres together, and in fact it requires that no directionality be exhibited in the bonding. This model does not work well when applied to structures built up of covalently bonded organic molecules, but it does work reasonably well for the crystals formed by ionic or metallic solids. 

Frequently, directionality is a property attributed to the covalent bond which supposedly is taken to be the cause of the resulting structures. However, as the success of the valence electron pair repulsion theory shows, there exists no need to assume any orbitals directed a priori. The concept of directed orbitals is based on calculations in which hybridization is used as a mathematical aid. The popular use of hybridization models occasionally has created the false impression that hybridization is some kind of process occurring prior to bond formation and committing stereochemistry. 

The compounds of carbon and silicon with hydrogen would be expected to be completely covalent according to these models, but the directionality of the bonds, which is towards the apices of a regular tetrahedron, is not explained by these considerations. Another of Pauling s suggestions which accounts for this type of directed covalent bonding involves so-called hybrid bonds. 

Leahy, D.E., Morris, J.J., Taylor, P.J. and Wait, A.R. (1992) Model solvent systems for QSAR. Part 3. An LSER analysis of the critical quartef . New light on hydrogen bond strength and directionality Journal of the Chemical Society-Perkin Transactions 2, 705-722. 

The success of the ligand-ligand repulsion model prompted its adoption as an element of a molecular mechanics program. In the resulting approach the valence angles around the metal ion are modeled solely by nonbonded interactions, using the usual van der Waals potential (for example, Eq. 2.9 kg = 0 in Eq. 2.7 Urey-Bradley approach)136. 6 Again, the fact that the electronic effects responsible for the directionality of bonds are not explicitly modeled here may seem questionable but extensive tests have shown the model to be reliable 371. An explanation for this apparent contra-... 

An important observation is that experimental structures may be reproduced satisfactorily with and without the angular terms involved in the hydrogen bonding interaction1481. Thus, irrespective of the fundamental reason for the deviation from linearity, the angular dependence may be modeled accurately either by van der Waals repulsion or by lone-pair directionality. 

The arrangement of atoms in a molecule is based on attractive and repulsive forces (see Fig. 2.2), as well as the directionality of the bonds, which is determined by the orientation of the bonding orbitals and their desire for maximum overlap. At first glance, the simple mechanical model (see Fig. 2.1) does not explicitly include specific electronic interactions. However, in developing a model that reproduces experimentally derived structural and thermodynamic data, it is inevitable that electronic factors are included implicitly to account for electronic effects responsible for some of the structural and thermodynamic variation present in the data used in the parameterization. Depending on the model used, the electronic effects may not be directly attributable to specific parameters. 

Leahy, D., J.J. Morris, and P.J. Taylor. 1992. Model Solvent Systems for QSAR. Part 3. An LSER Analysis of the "Critical Quartet " New Light on Hydrogen Bond Strength and Directionality. /. Chem. Soc. Perkin Trans. 2, 705-731. 

The dipole-dipole interaction model of the hydrogen bond shown above also accounts for the importance of the directionality of the hydrogen bond interaction. The energetically optimum arrangement occurs... 

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