Trigonal planar VSEPR structure

D—SiC, is tetrahedral. BrF4 is square planar. C2H2 is linear. TeF6 is octahedral. N03 is trigonal planar. If you are uncertain about any of these, Lewis structures and VSEPR are needed. 

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). 

Figure 3.13 also shows the structure for CH20. Because VSEPR theory predicts a trigonal planar geometry for the double bond and the two electron pairs on the oxygen, we will treat it as spr hybridized. Again, the picture is similar to ethene, with one CO sigma bond and one CO pi bond, but with both unshared electron pairs in... 

S4.8 If the N atom lone pair of (H3Si)3N is delocalized onto the three Si atoms, it cannot exert its normal repulsion influence as predicted by VSEPR rules. The N atom of (HsSifsN is trigonal planar, whereas the N atom of (HaOsN is trigonal pyramidal. The structures of (H3Si)3N and (H3C)3N, excluding the hydrogen atoms are ... 

Cyclic compounds like benzene are not the only ones with delocalized molecular orbitals. Let s look at bonding in the carbonate ion (CO3 ). VSEPR predicts a trigonal planar geometry for the carbonate ion, like that for BF3. The planar structure of the carbonate ion can be explained by assuming that the carbon atom is ip -hybridized. The C atom forms sigma bonds with three O atoms. Thus the unhybridized 2p orbital of the C atom can simultaneously overlap the 2p orbitals of all three O atoms (Figure 10.29). The result is a delocalized molecular orbital that extends over all four nuclei in such a way that the electron densities (and hence the bond orders) in the carbon-to-... 

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. 

Solution. The structure of BH3 is shown below, where the labels on the H atoms are imaginary. VSEPR theory predicts a trigonal planar electron and molecular geometry with bond angles of 120. The complete set of symmetry operations is , C3, 3, C2, C2, C2", tTh, S3, 83, c7v, CTv2> w3- The multiplication table is shown below. Once about half of the products were determined, the rearrangement theorem was used to help complete the multiplication table. 

The circled nitrogen atoms are predicted by VSEPR to be tetrahedral in geometry because each appears to have four areas of electron density three bonds and a single lone pair of electrons. However, in both cases, structural analysis has revealed that the atoms actually hae a trigonal planar geometry. 

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... 

These three electron groups can maximize their separation by assuming 120° bond angles in a plane—a trigonal planar geometry. Experimental observations of the structure of BF3 again confirm the predictions of VSEPR theory. 

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. 

Now both experimental measurements and VSEPR theory agree that the BX3 molecules in these substances are trigonal planar (Structure 9.2). As noted in Section 9.1.1, boron then has six outer electrons. 

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