Bent VSEPR structure

Thus, according to this alternate approach, SO2 belongs to the VSEPR class AX2E. Molecules of this type adopt a bent structure. 

The cu-bonding model provides a more complete and fundamental description of hypervalent molecules that are often interpreted in terms of the VSEPR model.144 In the present section we examine some MX species that are commonly used to illustrate VSEPR principles, comparing and contrasting the VSEPR mnemonic with general Bent s rule, hybridization, and donor-acceptor concepts for rationalizing molecular geometry. Tables 3.32 and 3.33 summarize geometrical and NBO/NRT descriptors for a variety of normal-valent and hypervalent second-row fluorides to be discussed below, and Fig. 3.87 shows optimized structures of the hypervalent MF species (M = P, S, Cl n = 3-6). 

The CNN unit is linear in A and B, but bent in C according to VSEPR. This is an example of how VSEPR can fail when comparing resonance structures. 

Compounds that appear to contain the [XeF]+ (1) and bent [Xe2F3]+ ions are known although the former is always strongly coordinated to a counterions such as SbF-6. Complex anions include XeF-5, XeF-7 and XeF2-g, the first of which has a unique pentagonal planar structure (3), as expected from VSEPR. 

The structures normally are those predicted by VSEPR theory. Triatomics are angular, BrF4 has a structure like SF4, while IF is octahedral C12F is bent and asymmetric, Cl—Cl—F+. ... 

Finally, water, H2O, has steric number 4 with two shared pairs and two unshared pairs on the oxygen atom. VSEPR theory predicts a bent structure, as a subcase of tetrahedral structure, with angles significantly less than the tetrahedral value of 109° due to repulsion between the two unshared pairs and the bonding pairs. Experimentally determined bond angles of 104° verify this prediction. 

Bent s rule is a useful tool in inorganic and organic chemistry. For example, it has been used to supplement the VSEPR interpretation of the structures of various nonmetal fluorides, and should be applicable to a wide range of questions on molecular structure. 

Thus CO2 has two "effective pairs" that lead to its linear structure, whereas O3 has three "effective pairs" that lead to its bent structure with a 120° bond angle. Therefore, to use the VSEPR model for molecules (or ions) that have double bonds, we use the same tools as those given on p. 430, but we count any double bond the same as a single electron pair. Although we have not shown it here, triple bonds also count as one repulsive unit in applying the VSEPR model. 

Based on VSEPR both SO2 and C1O2 will be bent structures, because of the lone pair on S and the lone pair + extra single unshared electron on Cl. The actual angle in SO2 is 129°, and that in CIO2 is 116°, the difference being due to the extra repulsion of the third nonbonding electron on Cl. 

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