Geometry of Molecules with More Than One Central Atom

Geometry of Molecules with More Than One Central Atom 

Thus far we have considered the geometries of molecules having only one central atom. We can determine the overall geometry of more complex molecules by treating them as though they have multiple central atoms. Methanol (CH3OH), for example, has a central C atom and a central O atom, as shown in the following Lewis structure  

Sample Problem 9.2 shows how to determine when bond angles differ from ideal values. 

Acetic acid, the substance that gives vinegar its characteristic smell and sour taste, is sometimes used in combination with corticosteroids to treat certain types of ear infections. Its Lewis structure is 

Determine the molecular geometry about each of the central atoms, and determine the approximate value of each of the bond angles in the molecule. Which if any of the bond angles would you expect to be smaller than the ideal values  

CHAPTER 9 Chemical Bonding II Molecular Geometry and Bonding Theories 

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GEOMETRY OF MOLECULES WITH MORE THAN ONE CENTRAL ATOM. 

So far we have discussed the geometry of molecules having only one central atom. The overall geometry of molecules with more than one central atom is difficult to define in most cases. Often we can only describe the shape around each of the central atoms. For example, consider methanol, CH3OH, whose Lewis structure is... 

Molecules in Which the Central Atom Has No Lone Pairs Molecules in Which the Central Atom Has One or More Lone Pairs Geometry of Molecules with More Than One Central Atom Guidelines for Applying the VSEPR Model... 

An interesting application of these principles is the prediction of CO dissociation routes on the closed-packed (111) surface of rhodium (see Fig. A.17). Two factors determine how the dissociation of a single CO molecule proceeds. First, the geometry of the final situation must be energetically more favorable than that of the initial one. This condition excludes final configurations with the C and the O atom on adjacent Rh atoms, because this would lead to serious repulsion between the C and O atoms. A favorable situation is the one sketched in Fig. A.17, where initially CO occupies a threefold hollow site, and after dissociation C and O are in opposite threefold sites. The second requirement for rupture of the CO molecule is that the C-0 bond is effectively weakened by the interaction with the metal. This is achieved when the C-O bond stretches across the central Rh atom. In this case there is optimum overlap between the d-electrons of Rh in orbitals, which extend vertically above the surface, and the empty antibonding orbitals of the CO molecule. Hence, the dissociation of CO requires a so-called catalytic ensemble of at least 5 Rh atoms [8,21,22]. 

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