"The Nitro Problem" is one of the most familiar issues in chemical informatics: How should a nitro group be best represented? Experimentally, the two oxygen atoms are equivalent, so it would make sense to depict them symmetrically. However, any way to depict them symmetrically will either violate the popular "octet rule" or force a double positive charge on the nitrogen -- neither of which is chemically reasonable [references???]. Conversely, any attempt to honor the octet rule results in oxygens that appear to be non-equivalent.
Similar problems arise for compounds based on sulfur, phosphorus, and related elements. Furthermore, all of these are fairly common functional groups, and cannot readily be pushed aside as "unusual" cases.
In the case of nitrogen compounds, a charge-separated depiction is recommended. While this does not preserve the visual appearance of equivalent oxygen atoms, we feel that it is acceptible by analogy with aromatic structures. Benzene is commonly drawn with three single bonds and three double bonds, but it is understood that all six bonds are acutally equivalent due to resonance delocalization between the two localized forms. Similarly, nitro functional groups should be considered to have both oxygens equivalent through a similar resonance pair.
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By analogy, other nitrogen oxides (and other chalcogenides, such as sulfides, etc.) should also be drawn in their charge-separated forms. Although this requires the addition of formal charges to an otherwise neutral structure, it can be justified by the observation that the nitrogen 3d orbital -- which would have to participate in any bonding scheme that resulted in five formal bonds to the nitrogen -- is energetically inaccessible to ground-state molecules [references???].
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| RECOMMENDED | AVOID | AVOID |
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Unlike with nitrogen, the d orbitals of phosphorus, arsenic, antimony, and bismuth compounds are readily accessible [references???]. Functional groups containing these elements are best depicted with normal single and double bonds and without the addition of extra formal charges, even though such representations will violate the octet rule.
| RECOMMENDED | AVOID | AVOID |
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This depiction style should be preserved even for anions. The four oxygen ligands are chemically equivalent, but that equivalence is commonly understood through resonance.
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As with phosphorus, the d orbitals of sulfur, selenium, and tellurium compounds are readily accessible [references???]. Functional groups containing these elements are also best depicted with normal single and double bonds and without the addition of extra formal charges, even though such representations will violate the octet rule.
| RECOMMENDED | AVOID | AVOID |
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Again as with phosphorus, this depiction style should be preserved even for anions. The four oxygen ligands are chemically equivalent, but that equivalence is commonly understood through resonance.
| RECOMMENDED | AVOID |
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