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Introduction

[should have one]

Delocalized bonds should be drawn to indicate delocalization

Delocalized bonds and other forms of π-bonding should be drawn to most clearly indicate that special bonding pattern. Depictions that imply a regular covalent bond -- and especially, depictions that show a regular covalent bond to each member of a delocalized system -- should be avoided.

As described in the discussion on aromaticity, systems that are truly delocalized should be depicted as such whenever possible [Harry: this conflicts with earlier recommendations, as the Kekule form of benzene has generally been recommended], generally by the use of curves rather than localized single and double bonds.

RECOMMENDED	RECOMMENDED	RECOMMENDED	RECOMMENDED	RECOMMENDED

AVOID	AVOID

From discussion with Alan McNaught, it sounds like IUPAC explicitly would like us to address at least some of the differences between drawings-for-print and drawings-for-electronic-use. Andrey points out that the next paragraph "seems too software oriented and is close to some drawing program guide or help file". He's right, but how can we present this issue (and many other similar ones) in a better way?

CAUTION: When working electronically, it is extremely important to specify a delocalized bond appropriately, according to the capabilities of the software program you are using. If the delocalized bond is specified inappropriately, the structure may be interpreted as two disjoint fragments, with the delocalized bond being interpreted as a normal bond to a carbon atom. In addition to losing the intended delocalized character, this misinterpretation will also add an additional CH₃ to the structures perceived formula for the "methyl group" at the center of the ring.

Coordination bonds

Bonds representing coordination should [something]. This needs more work.

Harry: Yes, needs more explanation! Why are these recommended or not?

RECOMMENDED (?)

AVOID	AVOID	AVOID (?)	AVOID (?)	AVOID (?)

Although it is recommended that atom labels always depict the correct number of hydrogens, that is especially important for coordination chemistry where bonding patterns may differ solely by the number of hydrogens involved:

	The electron pair on the nitrogen forms a bond to the metal. No hydrogens are lost.
	The NH2 loses H+ through ionization and then coodinates. This is less common with NH2 but happens and is common with groups such as the pyrrole nitrogen in porphyrin complexes. Common elements involved include N, O, and S (e.g., a cysteine may be present as R-S-M or R-S(H)-M.

It is similarly critical to indicate any hydrogens bound directly to the metal atoms.

also:

• boron hydrides
• agostic hydrogens

Polyatomic ions

Polyatomic ions add an additional complication. Not only do they often include coordination bonds, but any net charge is usually best represented as delocalized over the entire ion (or a large part of it), and not localized on a specific atom as in the case of most organic or metallic ions. [That's an awful description. Needs wordsmithing.]

When absolute structure is not important, an ion may be best represented by its formula. In accordance with [revised Red Book -- need reference], the formula should be placed in square brackets, and any charge should be reported as a superscript following the rightmost bracket. For more information about the formatting of formulas, see [revised Red Book -- need reference].

RECOMMENDED

If absolute structure is important, the ion should be drawn fully without concern for the delocalized charges. That structure should then be enclosed in large square brackets, and again any charge should be reported as a superscript following the rightmost bracket.

RECOMMENDED

Other structural depictions that involve localized charges are undesirable for a variety of reasons and should be avoided.

AVOID	AVOID	AVOID	AVOID

CAUTION: When working electronically, it is extremely important to specify a delocalized charges appropriately, according to the capabilities of the software program you are using. At the time of this writing, few chemical software packages are able to interpret delocalized charges, and both of the recommended depictions above would produce undesired results, resulting most likely either in a complete failure to interprety the structure or an incorrect interpretation. Given those weaknesses, software packages appear to most commonly prefer the structural depiction that localizes the negative charge on the metal atom. If you need to store structures of this type in an electronic format, please consult the documentation for the software package you are using for recommendations on the best methods to use with that package.

Depiction of delocalized and coordination bonds

Historically, delocalized and coordination bonds have been depicted in a variety of ways. Common usage now shows that such bonds are most commonly depicted as regular "plain" bonds, as shown above. A dashed bond is also seen, but less often.

With these recommendations, we suggest that delocalized and covalent bonds be best represented in most cases as plain bonds. Dashed bonds should be avoided in most cases because such bonds have been used to indicate stereochemical configurations rather than delocalization/coordination. The use of dashed bonds is still acceptible in cases where no stereochemistry is possible on the atom at either end of the dashed bond, but plain bonds remain preferred even in those cases. [Or do we want to deprecate dashed bonds in all cases?]

If a coordination bond is attached to an atom that does have a specified stereochemistry, then certainly a hashed or wedged bond depiction should be used instead, according to the other stereochemistry drawing conventions.

[Does anyone have good examples for any of these cases?]