[Vitnir]

Hello, first time poster here.

I got this problem sent to me, uranyl nitrate has two CAS numbers, why? The only thing I can come up with is that its two isotopes but I can find whether thats the case or not. Its not optically active I think so that cant be it. This link at national library of medicine shows what I mean and the compounds have identical structures as far as I can tell. If the link doesnt work, this is the CAS# 36478-76-9 and 10102-06-4 Edit: URL was shortened~Mitch

[Mitch]

I don't know. Although, I don't think its an uncommon phenomena.

[Vitnir]

You mean its simply a double entry in the CAS registry. I considered that but was confused with the slightly different entries for the two numbers.

[hmx9123]

There could be two entries--one for depeleted uranyl nitrate and one for undepleted uranyl nitrate.  I don't know for sure, though.

[jdurg]

I have no idea either, and because it will drive me to the point of insanity until I figure it out, I e-mailed CAS and asked them.  

[jdurg]

Props to the CAS group for their VERY quick response.  They sent me back an e-mail which I've posted below:

Dear Colleague,

Thank you for writing to us with your question. In case of the first
Registry Number, the coordination state of Uranium is specified (CN
Uranium, bis(nitrato-.kappa.O,.kappa.O')dioxo-, (OC-6-11)- (9CI)  (CA
INDEX NAME)
OTHER CA INDEX NAMES:
CN   Uranium, bis(nitrato-O,O')dioxo-, (OC-6-11)-

In the case of the second RN, the coordination state is different (CN
Uranium, bis(nitrato-.kappa.O)dioxo-, (T-4)- (9CI)  (CA INDEX NAME)
OTHER CA INDEX NAMES:
CN   Uranium, bis(nitrato-O)dioxo-, (T-4)-

Hence the two Registry numbers.

Best regards,

Tom T          

         CAS Customer Care
  Phone: (+1) 800-753-4CAS
         (+1) 614-447-3731
    Fax: (+1) 614-447-3751
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    Web: http://www.cas.org

So the two different numbers have to do with two different co-ordination complexes of the compound.  

[jdurg]

Here are the structures.  (Seen in the attachment below).  It just is a difference in the orientation of the groups.

[jdurg]

Structure #2

[AWK]

I think, that this explanation is to complex. Different CNs' concern
two different hydrates UO2.2H2O and UO2.6H2O;
and drawings of Jdurg not represent this differences because nitrate anions chelate uranyl groups probably twice in both hydrates.

Something is suspicious in the data for dihydrates from ICSD?
I will try to verify them as soon as possible.

[AWK]

Hexahydrate attached

[AWK]

I checked data for uranyl nitrates. I did not found data for anhydrous salt. All hydrates I found show the same coordination of UO2, hence the picture for dihydrate, though nice, is wrong. But another isomer of UO2(NO3)2(H2O)2 exists. It was found for polymorph of trihydrate in 2003.
In inorganic nomenclature, kappa in the name of a polydentate chelate complex indicating single ligating atom attachments of a polyatomic ligand to coordination centre. Hence both isomers represent "kappa, kappa" or kappa2 chelate. Two torsional isomers sketched by Jdurg represent (one) kappa complexes.

[Vitnir]

Uhh, thats what you get for not checking in for a few days  
Kudos for all the effort though I'm not sure I get it. I naturally looked at the structures but if you tilt your head you notice that the structure of 10102-06-4 is just rotated 90 degrees compared to 36478-76-9 so they look identical to me.
Different coordination states, what is that? The number of H2O coordinated to the salt of this compound? But if you dissolve it into water it would be the same compound again?
Sorry for the stupid question but I'm an analytical chemist.

[Mitch]

Being an analytical chemist is no excuse!

The structures are not identical since, the top one has empty coordination sites inbetween the two ligands.

[Vitnir]

You mean that the angle between N-O-U is 135 instead of 180? Maybe thats a significant clue to some but to me, who only considers unsaturated bonds to be rigid, it is not