Comments on: Halogen Bonding

By: Judy Hutchins

Judy Hutchins — Mon, 02 Mar 2009 11:18:12 +0000

Sorry “no” should say “known”. It’s 6:13am here and I haven’t slept since 27 hours ago.

By: Judy Hutchins

Judy Hutchins — Mon, 02 Mar 2009 11:16:55 +0000

Well, we might need to tell the new york city public school system this because in pre-college and even college chemistry classes, it’s no as Halogen bonds or H-bonds. So every year we get a lot of new students who will still be thinking that.

By: Mitch

Mitch — Thu, 28 Aug 2008 01:53:45 +0000

It was just an open question. Obviously, there is no 100% satisfactory answer.

By: excimer

excimer — Thu, 28 Aug 2008 00:39:35 +0000

It’s a misnomer now, but it didn’t seem to be when the term was coined a hundred years ago. Now it’s so deeply ingrained into the lexicon that it’ll be difficult to root out. A halogen “bond” is only called such because it’s analogous to the H-bond. Other interactions understood much later, like pi-stacking and cation-pi interactions, aren’t called bonds because of their dissimilarity to covalent bonds. Sorry mitch, history is getting in the way of your quest for accurate terminology.

So is a dative bond a bond, too? Is the only real bond a covalent bond? I’m not an emeritus professor yet, so the semantics don’t really seem relevant to progress in the field.

By: Phil

Phil — Wed, 27 Aug 2008 12:06:55 +0000

Thank you very much for your clarification.

By: Pierangelo Metrangolo

Pierangelo Metrangolo — Wed, 27 Aug 2008 09:11:44 +0000

I’ll try to give a short answer to some of the issues raised. Yes, it’s true, the definition of chemical bond, having a combination of orientation (alignment) and distance, applies also to halogen bonding since this causes interpenetration of VdW volumes of the interacting atoms by around 20% and occurs on the extension of the C-X bond where the sigma-hole is located (angle 160-180°).
Halogen bonds are comparable with hydrogen bonds, and span the energy range 1kcal-40kcal/mol (ionic X-bond). Halogen atoms bound to metal centers are not electrophilic (and then do not give XB) but behave as H- and X-bond acceptors (i.e. Lewis bases).
All these arguments can be found in the papers suggested in my web-site: http://nfmlab.chem.polimi.it

By: Phil

Phil — Mon, 25 Aug 2008 06:47:26 +0000

I wouldn’t apply the term “sigma-hole” to a metal because a copper ion is a lewis-acid and thus a non-directional electron acceptor. The sigma-hole of halogens has a very strong directionality.

By: Phil

Phil — Mon, 25 Aug 2008 06:45:03 +0000

I can only give you data from calculations. Lommerse et al. (JACS 1996, 118, 3108) have done ab initio calculations with chloro-cyanoacetylene and found a bonding energy of about 10 kJ/mol (2.4 kcal/mol). However, this is not a “typical” system.

Typical hydrogen bonds (gas phase) are in the range of 3-7 kcal/mol. Ionic H-bonds are much stronger. Still, in an aqueous (i.e. biological) system, halogen bonding can win a competition with hydrogen bonding because water can form H-bonds, too.

By: mitch

mitch — Sun, 24 Aug 2008 15:01:14 +0000

But how shallow a potential minimum are you willing to accept?

By: Egon Willighagen

Egon Willighagen — Sun, 24 Aug 2008 06:42:22 +0000

Phil, what is the typical bonding energy range for these kind of interactions? How much smaller than hydrogen bonds?

Mitch, to me a bond is a atom-atom interaction where the interaction has an energetic minimum resulting from repulsive forces at shorter distances, and attractive forces at larger distances (likewise, angles).