0.5 Molecular geometry and electron domain theory  (Page 5/6)

Observation 3: distortions from expected geometries

It is interesting to note that some molecular geometries( $C{H}_4$ , $C{O}_2$ , $HCCH$ ) are exactly predicted by the Electron Domain model, whereas inother molecules, the model predictions are only approximately correct. For examples, the observed angles in ammonia and watereach differ slightly from the tetrahedral angle. Here again, there are four pairs of valence shell electrons about the central atoms.As such, it is reasonable to conclude that the bond angles are determined by the mutual repulsion of these electron pairs, and arethus expected to be 109.5°, which is close but not exact.

One clue as to a possible reason for the discrepancy is that the bond angles in ammonia and water are both less than 109.5°. Another is that both ammonia and water molecules have lone pair electrons, whereas thereare no lone pairs in a methane molecule, for which the Electron Domain prediction is exact. Moreover, the bond angle in water, withtwo lone pairs, is less than the bond angles in ammonia, with a single lone pair. We can straightforwardly conclude from theseobservations that the lone pairs of electrons must produce a greater repulsive effect than do the bonded pairs. Thus, inammonia, the three bonded pairs of electrons are forced together slightly compared to those in methane, due to the greater repulsiveeffect of the lone pair. Likewise, in water, the two bonded pairs of electrons are even further forced together by the two lone pairsof electrons.

This model accounts for the comparative bond angles observed experimentally in these molecules. The valenceshell electron pairs repel one another, establishing the geometry in which the energy of their interaction is minimized. Lone pairelectrons apparently generate a greater repulsion, thus slightly reducing the angles between the bonded pairs of electrons. Althoughthis model accounts for the observed geometries, why should lone pair electrons generate a greater repulsive effect? We must guessat a qualitative answer to this question, since we have no description at this point for where the valence shell electronpairs actually are or what it means to share an electron pair. We can assume, however, that a pair of electrons shared by two atomsmust be located somewhere between the two nuclei, otherwise our concept of "sharing" is quite meaningless. Therefore, the powerfultendency of the two electrons in the pair to repel one another must be significantly offset by the localization of these electronsbetween the two nuclei which share them. By contrast, a lone pair of electrons need not be so localized, since there is no secondnucleus to draw them into the same vicinity. Thus more free to move about the central atom, these lone pair electrons must have a moresignificant repulsive effect on the other pairs ofelectrons.

These ideas can be extended by more closely examining the geometry of ethene, $C_2{H}_4$ . Recall that each H-C-H bond angle is 116.6° and each H-C-C bond angle is 121.7°, whereas the Electron Domain theoryprediction is for bond angles exactly equal to 120°. We can understand why the H-C-H bond angle is slightly less than120° by assuming that the two pairs of electrons in the C=C double bond produce a greater repulsive effect than do either ofthe single pairs of electrons in the C-H single bonds. The result of this greater repulsion is a slight "pinching" of the H-C-H bondangle to less than 120°.