VSEPR model trigonal planar structures

Show that the VSEPR model is consistent with a trigonal planar structure for SO3. 

The Group 13 atom in a trichloride or a trimethyl derivative is surrounded by three bonding electron pairs. According to the VSEPR model the three valence shell electron pairs surrounding the metal atom repel one another and occupy domains near the comers of an equilateral triangle with the metal atom at the center. The observed trigonal planar structures are thus in accord with the VSEPR model, but the model provides no explanation for the observation that the bonds in the trichlorides are shorter and weaker than in the monochlorides. 

The simplest Lewis formula for SO2 would place two electron pairs between the S and each of the two O atonos, one electron lone pair at the S and two electron lone pairs at each O atom S( 0 )2. This Lewis formula implies that the sulfur atom in SO2 accomodates five electron pairs in the valence shell (as in SF4). Similarly the simplest Lewis formula of the trioxide would place two electron pairs between the S and each O and two nonbonding electron pairs at each of the latter, S( 0 )3, indicating that the S atom in SO3 accomodates six electron pairs in its valence shell (as in SFe) [3]. Note that both the angular structure of SO2 and the trigonal planar structure of SO3 are in agreement with the VSEPR model. 

Section 8.13 molecular structure valence shell electron-pair repulsion (VSEPR) model linear structure trigonal planar structure tetrahedral structure trigonal pyramid trigonal bipyramid octahedral structure square planar structure... 

The SO bond distance in SO3 is 6 pm shorter than in SO the mean bond energy is 10% lower. This is the same mean bond energy decrease as that observed between SF2 and SFg. We conclude that on the basis of the experimental bond distance and bond energy, the three bonds in SO3 should be described as double. This implies that the S atom in SO3, like the S atom in SFg, has six electron pairs in the valence shell. According to the VSEPR model, repulsion between the three domains each containing two electron pairs should lead to the trigonal planar structure that is observed. 

A boron trifluoride molecule, BF3, has the Lewis structure shown in (5). There are three bonding pairs attached to the central atom and no lone pairs. According to the VSEPR model, as illustrated in Fig. 3.4, the three bonding pairs, and the fluorine atoms they link, lie at the corners of an equilateral triangle. Such a structure is trigonal planar, and all three F—B—F angles are 120° (6). 

The fact that heptacoordinated species in their ground states exMbit pentagonal bipyramidal structures with an unpuckered equatorial plane, cannot be rationalize by VSEPR theory (7,2) in terms of a repelling points on a sphere (POS) model which should result in either a monocapped octahedron or a monocapped trigonal prism. Furthermore, it cannot be explained by conventional bonding schemes involving localized electron orbitals of die central atom to enforce the coplanarity of a central atom and five equatorial ligands. The best explanation to account for this planarity is the... 

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