# What does the colon in µ-η2:η2-O2 mean?

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?

• Can we have a source? – Jan Mar 21 '17 at 13:36
• $\mu$ 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. – orthocresol Mar 21 '17 at 13:37
• 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-10.2.3.1, and IR-10.2.5.1 (notably the text accompanying Example 21 on p 222) in the 2005 Red Book mention this use of colons. – orthocresol Mar 21 '17 at 13:37
• – mhchem Mar 21 '17 at 14:29
• 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. – orthocresol Mar 21 '17 at 15:22

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

IR-10.2.5.1 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-10.2.3.1 and IR-10.2.3.4) 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.

(…)

Examples:

21.

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.