VSEPR theory, determination bond orders

Discuss the nature of the bonding in the nitrite ion (NO2). Draw the possible Lewis resonance diagrams for this ion. Use the VSEPR theory to determine the steric numbeg the hybridization of the central nitrogen atom, and the geometry of the ion. Show how the use of resonance structures can be avoided by introducing a de-localized 77 MO. What bond order does the MO model predict for the N—O bonds in the nitrite ion ... 

Determining the molecular geometry becomes complicated when the central atom has both lone pairs and bonding pairs. Three types of unequal repulsive forces are present in such molecules. In general, the order of repulsive forces as predicted by VSEPR theory is... 

N , and N and (iii) the bond orders for N —N and N —N . Finally, comment on whether or not the calculated structural parameters are consistent with the VSEPR theory and determine the structure of anion AsF J using this theory. 

Although molecular orbital theory is in many ways the most powerful of the bonding models, it is also the most complex, so we continue to use the other models when they do an adequate job of explaining or predicting the properties of a molecule. For example, if you need to predict the three-dimensional shape of an AB molecule on an exam, you should draw its Lewis structure and apply the VSEPR model. Don t try to draw its molecular orbital diagram. On the other hand, if you need to determine the bond order of a diatomic molecule or ion, you should draw a molecular orbital diagram. In general chemistry, it is best to use the simplest theory that can answer a particular question. 

If we do not use hyperconjugation, how are we to describe where the "extra" electrons go This question arises because we seem implicitly to think about bonding in terms of hybridization of atomic orbitals. Thus, in order to describe bonding in the methane molecule consistent with its tetrahedral geometry we introduced the concept of sp hybrid orbitals. That is, the hybridization was introduced because of the geometry. The geometry of a molecule can be determined only experimentally or estimated by using VSEPR theory. 

The controversy as to how best to write Lewis structures will no doubt continue in the chemical literature, but you should not be too dismayed by this situation. Our approach to depicting the electronic structure of a molecule is based on the simplest Lewis structure and its concomitant use in determining the shape of a molecule through VSEPR theory. In order to probe more deeply into the nature of a chemical bond—for example, to understand experiment results, such as bond enthalpy values— we must analyze a computed electron density map for that molecule rather than rely just on the Lewis structure. 

Molecules of noble gas compounds have shapes in excellent agreement with VSEPR theory, although to form two-electron bonds, xenon must promote valence electrons to higher-energy orbitals such as 5d. This model can be avoided by a molecular orbital treatment which, in Xep2, forms three-centre bonds from xenon 5p and fluorine 2p orbitals. The Xe—F bonds then have an order much less than one. The experimentally determined bond lengths are in better agreement with the two-electron, two-centre bonds of VSEPR theory. 

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