In the formula $\ce{[nBu4N]2[(UO2)2(\mu-\eta^2:\eta^2-O2)(NO3)2(\mu-Au(CN)2)2]}$, what does the colon stand for? Is this an official notation?

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    $\begingroup$ Can we have a source? $\endgroup$
    – Jan
    Commented Mar 21, 2017 at 13:36
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    $\begingroup$ $\mu$ means that the $\ce{O2}$ ligand is bridging two metal centres, i.e. it is bound to two different metal atoms/ions. $\eta^2$ means that the $\ce{O2}$ ligand is acting as a bidentate ligand (i.e. both oxygens are bonded to the metal). $\eta^2\text{:}\eta^2$ means that the $\ce{O2}$ ligand is acting as a bidentate ligand towards both metal centres. $\endgroup$ Commented Mar 21, 2017 at 13:37
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    $\begingroup$ So, it looks something like this (the structure of oxyhemocyanin, bottom): web.tock.com/kalee/chem32/spec/Image1.gif There, $\ce{O2}$ (in the middle) is acting as a bidentate ligand towards two copper centres. In your compound I presume it is acting as a bidentate ligand towards two U centres. Now, as to whether it is an "official" notation, IR-2.5.2, IR-, and IR- (notably the text accompanying Example 21 on p 222) in the 2005 Red Book mention this use of colons. $\endgroup$ Commented Mar 21, 2017 at 13:37
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    $\begingroup$ Source: chat.stackexchange.com/transcript/message/36064040#36064040 $\endgroup$
    – mhchem
    Commented Mar 21, 2017 at 14:29
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    $\begingroup$ Good catch. I looked through again and can't find any examples of the use of the colon in a name. However, as far as I can tell it is permissible to use both μ and η in formulae, see IR-4.6.5 (p 67) and Table V (p 259). The colon is not a structural descriptor such as μ and η; it simply serves to delineate two structural descriptors. Just based on this, I would assume that the use of : in formulae is allowed, but I acknowledge it isn't really hard proof. $\endgroup$ Commented Mar 21, 2017 at 15:22

1 Answer 1


This notation is mentioned in the IUPAC Nomenclature of Inorganic chemistry, Recommendations 2005 (‘Red Book’):

IR- The eta (η) convention

The special nature of the bonding of unsaturated hydrocarbons to metals via their π-electrons has led to the development of the ‘hapto’ nomenclature to designate unambiguously the unique bonding modes of the compounds so formed. (…) The Greek symbol η (eta) provides a topological description by indicating the connectivity between the ligand and the central atom. The number of contiguous atoms in the ligand coordinated to the metal is indicated by a right superscript numeral, e.g. η3 (‘eta three’ or ‘trihapto’), η4 (‘eta four’ or ‘tetrahapto’), η5 (‘eta five’ or ‘pentahapto’), etc.


If an unsaturated hydrocarbon serves as a bridging ligand, the prefix μ (see Sections IR- and IR- is combined with both η and κ, where necessary. The colon is used to separate the locants of the bridging ligand which indicate binding to different metal atoms.




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(This convention is not used for unsaturated hydrocarbons only, there is also a more complicated example (25) with carbonyl (CO) ligand.)

Use of this notation for formulae does not seem to be explicitly mentioned or “allowed”. If it is not an omission in the rules, I think that such construct could be called a name-formula hybrid, which might useful for detailed description the discussed structure detail in a shortest way possible.

The $\ce{[^{n}Bu4N]2[(UO2)2(\mu-\eta^2:\eta^2-O2)(NO3)2(\mu-Au(CN)2)2]}$ polymeric complex is described in [1], which talks about $\ce{U-O2-U}$ linkage with bidentate peroxo ($\ce{O2^{2-}}$) unit.


  1. Brown, M. L.; Ovens, J. S.; Leznoff, D. B. Dicyanoaurate-Based Heterobimetallic Uranyl Coordination Polymers. Dalton Transactions 2017, 46 (22), 7169–7180.

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