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Question for chemists about lanthanide-carboxyl structures and their formation

  1. Jul 29, 2005 #1
    In a lab I work at at school we are trying to create aggregates of the lanthanide gadalinium along with carboxyl groups which are attached to a long (20 or so) carbon chain. The problem is we are getting polymers of lanthanides with carboxyls instead of a large number of universally sized aggregates of a single gadolinium atom bonded to three carboxyl groups, which is what we want.

    Does anyone have any advice, input or can you point me in the direction of any books or scientific articles on carboxyl/lanthanide structures and their formation? I would appreciate it as right now I'm trying to learn about the nature of whats going on and how to create gadolinium with 3 carboxyls on it instead of a massive polymer.

    We are trying to create gadolinium 3+ atoms with metalloligand bonds to three carboxyl groups in the hope that this will fulfill the f-shell of the Gd.

    My inorganic chemistry is a bit rusty though, and I learned more about S and P shells, not F shells. I know lanthanides have 7 electron shells (s=1, p=3, d=5, f=7) and I'm pretty sure that a gadolinium 3+ atom would have 6 electrons in its shell. From what I remember you want as many electrons to be unpaired as possible, but I thought that a 2+ charge was considered the de facto number of electrons which woudl mean that Gadolinium 3+ already has 6 unpaired electrons, and would prefer to only make one bond. Some Gd-ligand complexed like Gd-DTPA
    or Gd-DOTA
    show 8 bonds. You'd assume that would happen as Gd in a 3+ state has room for 8 electrons.

    However some pictures of Gd-DTPA or Gd-DOTA show the Gd bonded to a water molecule as well, making 9 ligands. Where do the extra water electrons go? Where do they fit?

    In my view, maybe we are going about this all wrong for a few reasons.

    1. Gadolinium 3+ will only accept 1 electron to gain stability since it'll have 7 unpaired electrons. it can hold up to 8. Trying to add 3 ligands (the professor thinks that Gd 0 is the most stable form) may be unwise as we'll have 2 sets of paired electrons and 5 unpaired electrons. I don't have any idea how to draw up the MO for an f shell bond though. I barely understand how to do it for S and P shells.
    2. Gadolinium can bond with up to 8 things, I don't understand F block inorganic chemistry nearly well enough to know what happens to encourage metalloligand bonding, but I just don't see how it is realistic to have only 3 bonds. Either we'd have 1 bond or 8 bonds.

    Would we be better off trying 1+ Neodymium, 2+Promethium, 3+ Samarium or 4+ Europium instead of 3+ Gadolinium? Are any of these molecules stable or affordable? Are things like 5+ Gadolinium stable or existant? Is having a charge of 0 more important than having the maximum number of unpaired electrons?

    Would doing the synthesis in an acidic environment help any? From what I've gathered by talking to the other professors there is resonance in the carboxyl oxygens, making 2 possible ligands on one molecule. If we left the hydrogen on and had a carboxylic acid would that still be able to metaolligand bond with the Gd?

    Keep in mind that my chemistry isn't great, so there can be problems with my view on this subject of f shells and how they relate to metalloligand bonding but any info is helpful.
  2. jcsd
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