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What do you mean by a pseudohalogen/pseudohalide and how do you tell whether a molecule/ion is a pseudohalogen/pseudohalide ?

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Pseudohalides are "fake halides." Their chemistry bears some resemblance to true halides ($\ce{F-}$, $\ce{Cl-}$, $\ce{Br-}$, $\ce{I-}$).

Examples of pseudohalides include $\ce{CN-}$ and $\ce{N3-}$. These are generally weak Lewis bases that bear a formal negative 1 charge.

In my opinion, it is rather subjective as to what can be labeled as a pseudohalide. In general, it is in the eye of the beholder, and for a certain set of chemical circumstances.

In short, there is no "dividing line" for being a pseudohalide. If we assume that halides undergo certain (characteristic) types of chemistry, and the species X in question also generally does those same types of chemistry with a similar mechanism, then you may be able to label X a "pseudohalide."

(Note that within the true halides themselves, they can have very different characteristics, and so there is a bit of a moving target for what constitutes a pseudohalide.)

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  • $\begingroup$ But F-,Cl- tend to make low-spin complexes and CN- ion tends to make a high-spin complex. Now considering this difference isn't 'pseudohalogen' a misnomer ? $\endgroup$ – kesari May 29 '13 at 15:41
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    $\begingroup$ Yes, it is probably a misnomer in the context of coordination chemistry, that you cited here. But, it may have certain aspects of acid/base chemistry that look like halogens. BTW, I think it's kind of a silly concept: pseudohalides. It's just asking for a fight as to whether group X is enough like any/all/some halides in all reactivities/spectroscopies to be called "pseudohalide." Truth is, X is a halide or it is not. $\endgroup$ – Eric Brown May 29 '13 at 18:49
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A pseudohalogen is an anion with similar properties to a halogen, but that isn't a halogen. The true halogens are of course the group one column to the left of the noble gases; fluorine, chlorine, bromine, iodine, and astatine, as well as the theoretical ununseptium.

The pseudohalogens are generally -1 compound anions with stable structure (typically featuring multiple bonds) that form compounds with similar reaction behaviors and properties as halides, and can be used as general substitutes in reactions with similar mechanisms. These properties include being weak bases by themselves that form strong acids when protonated, forming neutral salts in combination with metals, and oxidizing electropositive elements. Common pseudohalogens include cyanide/isocyanide, cyanate/isocyanate, azide, their sulfur analogs such as thiocyanate, and complexes such as tetracarbonyls.

However, pseudohalogens are not always identical. Take for instance sodium chloride, and the pseudohalogen analog sodium azide. While the former is relatively harmless table salt, consumed in tens of grams every day, the latter is so generally toxic to life that biochem labs use it as a reference poison (and to make sure whatever nasties they've been growing in vitro for an experiment are good and dead before disposing of the medium). Similarly, while the human body needs a decent dose of chlorine anion as an electrolyte, and smaller amounts of iodine and even bromine are essential trace elements, most of the pseudohalogens are quite toxic, including cyanide, azide and carbonyl complexes. Thiocyanates and isothiocyanates are essential functional groups and anions in human biochemistry, so not overtly toxic like azide, but - First Lesson in Toxicology - too much of anything is bad.

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  • $\begingroup$ Can one say that the diatomic interhalogen compounds ,eg. ClF are pseudohaogen compounds ? $\endgroup$ – kesari May 29 '13 at 15:38
  • $\begingroup$ I would say not, first because interhalogens are typically not ions (they end up sharing their lone electron forming an electrically neutral compound) and because this structure typically isn't preserved in any reaction; the two halogens would much rather oxidize anything else than each other. However, there are halides of pseudohalogens, such as chlorine and fluorine azides. While similarly reactive, the stable pseudohalogen group is typically preserved in an exchange or oxidation. $\endgroup$ – KeithS May 29 '13 at 16:13

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