Talk:Ligand

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[edit] Merge proposal

This discussion is being held at Talk:Ligand (biochemistry). Single word comment moved there. Walkerma 02:49, 17 December 2005 (UTC)

The word ligand is used very differently in biochemistry and in chemistry. It is important to keep this as a branch point. anyone searching for ligand would come in here and click on the biochemistry side or the inorganic chemistry side. They can be directed to the right place. This should not be moved permanently to biochemistry {ashoka.samuelson@gmail.com}

As a chemistry undergraduate I found the ligand (chemistry) page very clear and extreamly useful when researching for EDTA titration information. Having looked at the ligand (biochemistry) article I think I would have found it confusing if the two had been merged. I am happy as things are - with a link between the pages. Thanks for the excellent service. (contributed by 86.132.215.214)

Thank you for the constructive criticism and positive feedback. It is quite helpful. User:Ceyockey (talk to me) 16:56, 19 February 2006 (UTC)

Since the consensus is to keep these articles apart, I think the merge proposal should be removed. --Scott 12:50, 25 March 2006 (UTC)

[edit] Removed sentence

The charge on the central atom (called the oxidation number) constrains the number of ligands that may be bonded because each type of ligand donates a characteristic number of electrons, and there is a requirement for overall neutrality. This sentence was removed. It is strictly true that neutrality is required for a molecule overall, but the complex may be a complex ion (positive or negative) and most transition metals bond octahedrally regardless of the charges of the ligands or their own oxidation states. I see what the author of the above sentence was getting at, but how could it be better said? Srnec 21:59, 19 April 2006 (UTC)

[edit] Cleanup

I moved a lot of parts from Complex (chemistry), which were merely descriptions of ligands, to this page, I do believe that some could serve a function here, but there is quite some redundancy. I will try to clean-up the mess later (but feel free to mess around with it), for now I am busy with Complex (chemistry). --Dirk Beetstra 21:24, 16 May 2006 (UTC)

proposed deletions:
"Reactions" on outer and inner-sphere
"Geometry" which should be under coordination or complex chem
"Older classification of isomerism" seems pretty old fashioned.
"Spectroscopy" is an effort to deal with CFT and should be moved to complex chem.--Smokefoot 22:28, 17 May 2006 (UTC)

Smokefoot, I fail to understand the rationale for most (perhaps all) of your most recent edits. For example:

  • Why was "bond" changed to "bind"? "Bond" is the proper verb in this context.
  • Why the removal of terms like "central atom" and "polydentate" which are informative and correct?
  • Perhaps I got hapticity and denticity wrong, why then did you only add to the confusion?
  • What's wrong with a sentence like "The larger the macrocycle, the more rigid, and the more coordinate sites to which it bonds, the more inert the complex"?
  • While I understand why the isomerism section was reformed (based on more modern defintions), I don't think it made it more comprehensive and comprehensible.

Finally, the sections you want to delete are there so that someone finds them when they come to this article. They are not in-depth. They are here for comprehensiveness (so that nothing of importance is completely left out, leaving the reader to search it out some more). Srnec 00:15, 18 May 2006 (UTC)

Smec: thanks for your comments and glad to see that someone was watching. I also found the article almost inpenetrable and apparently Beestra had trouble with it also. The article still needs work for sure. I just ran out of time. If you feel strongly about "bond" vs "bind" then go for it. I prefer bind because the term avoids electronic implications - i.e. covalency vs ionicity.
  • Polydentate does not include bidentate, I dont think. So I rephrased that (poly means more than 2).
  • Re: kappa vs hapto. polydentate ligands are typically described using kappa convention, although this is less religiously used than the hapto approach. So I added to the confusion because what was there was deemed incorrect.
  • the isomerism part gave the reader the impression that isomerism for coordination compounds are somehow different from isomerism in other kinds of molecules.
  • large floppy macrocycles are not better ligands, so perhaps you wish to rephrase this bit.
  • the concepts related to "ionization isomerism" and "solvate isomerism" are archaic and probably should be deleted because they misinform. These ideas date from ancient times when structures were poorly understood. These ideas are rarely employed in current coordination chemistry, so it seems ill-advised to legitimize them in a very short thumbnail. No doubt they lurk in textbooks somewhere still.

Take a whack at fixing it up! We can work together as I have done with others.--Smokefoot 02:36, 18 May 2006 (UTC)

One more thing Smec: would you like to merge the two ligand list? Conceivably this site could be the place people go to find ligands.--Smokefoot 03:06, 18 May 2006 (UTC)

I have moved some things to Complex (chemistry) (reactions, geometry (which was already available on the other side), isomerism (needs cleanup on the other side). I still have a bit of trouble with strong/weak field ligands. The spectroscopy belongs on the other side, where the strength has reason to be on this end... I will edit on when I have time today. About the large macrocycles, take care not to write it in a 'by definition' way, that a ligand is multidentate does not by definition mean that it will bind better than the loose ligands (I have seen some tridentate 'ligands' around, which just seem to refuse in all cases to act as a ligand. There is more to it than multidenticity, the 'hardness' of the ligand has to match the metal, the geometries have to be OK (of the free orbitals on the metal in combination with how the glove fits the hand), and some metals have a tendency to prefer certain symmetries. --Dirk Beetstra 07:17, 18 May 2006 (UTC)

[edit] Denticity vs. hapticity

Is the following sentence correct:

Hapticity and denticity should not be confused. An η5 ligand may be κ1 if all five bonding atoms of the ligand bond to one coordination site (as in cyclopentadienyl).

Srnec 17:07, 18 May 2006 (UTC)

The κ notation has me always confused, I think that a Cp is η5 but also κ5 .. but N,N,N',N'-tetramethyl-1,2-diaminoethane (TMEDA) would have to be described as η2, κ1(N), κ1(N) .. but maybe then with Cp it might as well be κ1(C) (for each C) .. but I would have to look it up, I know that the κ-notation is in general omitted, but I believe that it is not correct. There must be a IUPAC-document online somewhere, where this is part of the description.
Nontheless, hapticity and denticity are indeed different things, I would describe Cp with a hapticity of 5, but a denticity of 1 (in general, there are some strange exceptions). --Dirk Beetstra 18:29, 18 May 2006 (UTC)

So the sentence is correct? Good. Also, to answer a question above, the lists are convenient separate so that people can identify ligands based on different properties, or find a ligand of a given property. Srnec 03:18, 19 May 2006 (UTC)

[edit] Database problem

Seems that the page came true, removed the dump again (illegible anyway) --Dirk Beetstra 22:27, 24 May 2006 (UTC)

[edit] table(s) of common ligands - please expand

I have put the remainder of the ligands in a table, which is far from complete, so I have added an 'expand' to it. Things that need to be done:

  • Some of the ligands can be placed in the upper table (the one sorted by field-strenght). So which can be placed there, should be moved to their appropriate place.
  • There are some common ligands missing, so they have to be added.
  • Table needs more filling, most formulae are missing, e.g.
  • I have no clue about table-formatting, I think the 2 tables should be similar, can someone help with that?

Hope you can help. Cheers! --Dirk Beetstra 13:17, 29 May 2006 (UTC)

[edit] Disambiguation

There are now two statements above the page, where the word ligand is used in another sense than the inorganic way, should 'ligand' not be a disambiguation page, pointing towards ligand (biochemistry), ligand (pharmacology) and ligand (chemistry) (the latter should then contain the information now on ligand)?? --Dirk Beetstra 09:03, 3 June 2006 (UTC)

I created ligand (disambiguation), which points to the respective pages, I now see, that many pages linking to ligand (many of these links) actually should point to ligand (biochemistry). --Dirk Beetstra 10:06, 3 June 2006 (UTC)

[edit] You can't have it both ways, i.e. L = donor, covalent, and acceptor

The current article (July 9, '06) states: "In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with one or more central atoms or ions (these ligands act as a Lewis base. Fewer examples exist where a molecule can be described as a ligand that accepts electrons from a Lewis base (hence, the ligand acts as a Lewis acid)." Whether we like it or not, the second sentence contradicts the first. We can't begin the article by saying in effect " ligands are Lewis bases, unless they are Lewis acids, and oh, sometimes they donate one electron to make covalent bonds" - which is what it currently says. I dont have any great ideas about how to resolve the current wide-open definition.

My recommendation would be to admit that the nomenclature is imperfect but that generally, ligands are considered to be the Lewis base component of an adduct. Thus BF4- has four fluoride (vs fluorine) ligands. --Smokefoot 18:12, 8 July 2006 (UTC)

I see your problem, but to add to this discussion .. isn't BF3 a ligand for F- in BF4- ... ?? I'll think about it .. --Dirk Beetstra T C 23:45, 8 July 2006 (UTC)
Welcome back. BF3 would not be considered to be a ligand for F- in BF4- ... Its the other way around. at least around here. --Smokefoot 00:23, 9 July 2006 (UTC)
Thanks! Yes, the general way is to define the fluoride to be the ligand. But, when looking at the coordination chemistry of the fluoride, BF3 is the ligand ... But I think we will have to have a look in textbooks for this one, how it is defined. To my feeling, people think of ligands as the things coordinating to a metal centre, and metal centres in general want electron density, and therefore most of the molecules considered ligands are Lewis bases, i.e. electron donating things. But when applying the same to organics .. H is the ligand for C in methane? Or would you then argue that CH3- is the ligand for H+, just because the ligand has to donate electrons...?? --Dirk Beetstra T C 08:17, 9 July 2006 (UTC)
  • I am okay with a ligand being very general, the central atom or a peripheral one. It's the opening paragraphs taht are awkward, once one lauches on coordination chem and organometallics, we all agree on what a ligand is and looks like, something that is attached to something else.--Smokefoot 14:01, 9 July 2006 (UTC)
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