Question

Sometimes more than one atom arrangement is possible, these are called geometric isomers. For the molecules below sketch all the possible geometric isomers. Then keeping in mind that nonbonding pairs on the central atom will try to get as far away as possible from both other bonding and nonbonding pairs, predict which will be the prefferred geometry in nature. Justify your choice. Name the molecular geometry of the prefferred structure.

SF4(2 isomers)

BrF3(3 isomers)

XeF4(2 isomers)

Answer 1

SF4 exhibits an unusual "see-saw" geometry.

Square planar structure

This can be rationalized by VSEPR theory by first noting that the best way to arrange the sulfur's five electron pairs is in a trigonal bipyramid, and second by postulating that the non-bonded electron pair takes up more space than the bonded pairs. What VSEPR theory does not tell us is whether there are alternative "stable" structures for SF4, for example, trigonal-base-pyramid and square-planar structures. X3E2: BrF3 1. The bromine atom has seven valence electrons, and each fluorine has seven valence electrons, so the Lewis electron structure is Once again, we have a compound that is an exception to the octet rule. 2. There are five groups around the central atom, three bonding pairs and two lone pairs. We again direct the groups toward the vertices of a trigonal bipyramid. 3. With three bonding pairs and two lone pairs, the structural designation is AX3E2 with a total of five electron pairs. Because the axial and equatorial positions are not equivalent, we must decide how to arrange the groups to minimize repulsions. If we place both lone pairs in the axial positions, we have six LP–BP repulsions at 90°. If both are in the equatorial positions, we have four LP–BP repulsions at 90°. If one lone pair is axial and the other equatorial, we have one LP–LP repulsion at 90° and three LP–BP repulsions at 90°: Structure (c) can be eliminated because it has a LP–LP interaction at 90°. Structure (b), with fewer LP–BP repulsions at 90° than (a), is lower in energy. However, we predict a deviation in bond angles because of the presence of the two lone pairs of electrons. 4. The three nuclei in BrF3 determine its molecular structure, which is described as T shaped. This is essentially a trigonal bipyramid that is missing two equatorial vertices. The Faxial–Br–Faxial angle is 172°, less than 180° because of LP–BP repulsions XeF4: