Resonance with formal charges

The ionic representation of the ylides in Figure 9.1 shows only one of two conceivable resonance forms of such species. In contrast to the N atom in the center of the N ylides, the P or S atoms in the centers of the P and S ylides (Figure 9.1) may exceed their valence electron octets and share a fifth electron pair. For P and S ylides one can therefore also write a resonance form with a C=P or C=S double bond, respectively these are resonance forms free of formal charges (Figure 9.1). For the sulfoxonium ylide there is a second resonance form in which the S atom exceeds its valence electron octet however, this does contain formal charges. Resonance forms of ylides in which the heteroatom exceeds its valence electron octet are called ylene resonance forms. The ene part of the designation ylene refers to the double bond between the heteroatom and the deprotonated alkyl group. 

If one resonance form looks more like the resonance hybrid than the others, it weights the average in its favor. One way to select the more important resonance form is by determining each atom s formal chaise, the charge it would have if the bonding electrons were shared equally. Let s examine this concept and then see how formal charge compares with oxidation number. 

When different resonance structures are possible, some giving the central atom in a compound an octet and some an expanded valence shell, the dominant resonance structure is likely to be the one with the lowest formal charges. However, there are many exceptions and the selection of the best structure often depends on a careful analysis of experimental data. 

Structural isomers are molecules that have the same formula but in which the atoms are connected in a different order. Two isomers of disulfur difluoride, S2F2, are known. In each the two S atoms are bonded to each other. In one isomer each of the S atoms is bonded to an F atom. In the other isomer, both F atoms are attached to one of the S atoms, (a) In each isomer the S—S bond length is approximately 190 pm. Are the S—S bonds in these isomers single bonds or do they have some double bond character (b) Draw two resonance structures for each isomer, (c) Determine for each isomer which structure is favored by formal charge considerations. Are your conclusions consistent with the S—S bond lengths in the compounds ... 

In the examples presented so far, all the resonance structures are equivalent, but resonance structures are not always equivalent. Resonance structures that are not equivalent occur when Step 5 requires shifting electrons from atoms of different elements. In such cases, different possible structures may have different formal charge distributions, and the optimal set of resonance structures includes those forms with the least amount of formal charge. Example treats a molecule that has near-equivalent resonance structures. 

This structure is isoelectronic with (Si03)36 and (P03)33-. Gaseous S03 has a trigonal planar structure that has several contributing resonance structures. When the structure is drawn with only one double bond, the sulfur atom has a +2 formal charge that is relieved by stmctures having two double bonds. Therefore, multiple bonding is extensive. 

Write the Lewis structures, including resonance structures where appropriate, for (a) the oxalate ion, C2042- (there is a C—C bond with two oxygen atoms attached to each carbon atom) (b) BrO+ (c) the acetylide ion, C22-. Assign formal charges to each atom. 

Resonance. Consider the dot formulas for the nitrate ion. One may think of several ways of pairing up the twenty-four electrons in this ion. The formulas with formal charges are given below. 

Some atoms, even in covalent compounds, carry a formal charge, defined as the number of valence electrons in the neutral atom minus the sum of the number of unshared electrons and half the number of shared electrons. Resonance occurs when we can write two or more structures for a molecule or ion with the same arrangement of atoms but different arrangements of the electrons. The correct structure of the molecule or ion is a resonance hybrid of the contributing structures, which are drawn with a double-headed arrow () between them. Organic chemists use a curved arrow (O) to show the movement of an electron pair. 

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