# How to name this quintuply-bonded chromium dimer?

Depicted below is a chromium compound with a metal–metal quintuple bond. It boasts two identical bridging ligands, derivatives of 1,3-diphenylbenzene. The aim of this question is to figure out

• the proper name.

Proper means without losing information. When answering, application of the 2005 Red Book and 2013 Blue Book is desired.

Effort

1. Identify central atom, derive ending: chromium
2. Naming unsubstituted ligand, derive ending: 1,3-diphenylbenzene $\longrightarrow$ 1,3-(diphenyl)phenyl
3. Allow for chromium's probably higher priority $\longrightarrow$ 2,6-(diphenyl)phenyl
4. Add in the isopropyl groups $\longrightarrow$ 2,6-{[2,6-di(1-methylethyl)]diphenyl}phenyl
5. Look for stereoisomers of ligand: probably none
6. Count central atoms: two
7. Identify and denote ligand connectivity to central atoms: 1κ$C^1$,2κ$C^7$
8. Exploit symmetry, count ligands, prefix di already used: two $\longrightarrow$ bis
9. Combine: bis(2,6-{[2,6-di(1-methylethyl)]diphenyl}phenyl-1κ$C^1$,2κ$C^7$)chromium
10. Account for the bridging nature of ligands: add μ
11. Notice metal–metal bonding: add $(Cr$—$Cr)$
12. Probably (***) count number of bonds between metals: five $\longrightarrow$ $(5\ Cr$—$Cr)$
13. Combine:

bis(μ-2,6-{[2,6-di(1-methylethyl)]diphenyl}phenyl-1κ$C^1$,2κ$C^7$)chromium$(5\ Cr$—$Cr)$

1. Change parentheses to allow for two chromium atoms: add []
1. Final proposed name (***):

bis[(μ-2,6-{[2,6-di(1-methylethyl)]diphenyl}phenyl-1κ$C^1$,2κ$C^7$)chromium]$(5\ Cr$—$Cr)$

• Is this correct? Where have I gone wrong?

*** Point 12 conflicts with the Red Book. See IR-10.2.3.5 which says

For the purpose of nomenclature, no distinction is made between different metal–metal bond orders.

This is at odds with imposed criterium of preserving information, hence was ignored. ***

Assembling the chemical formula will most likely be covered in a follow-up question.

It was a great effort. However, in the proposed name

bis[(μ-2,6-{[2,6-di(1-methylethyl)]diphenyl}phenyl-1κC1,2κC7)chromium](5 Cr—Cr)

I can see several issues:

• In you point 12: Forcing the metal-metal bond multiplicty in the name is interesting, however I wouldn't go against the rules. See the example with the multiple (triple) Cr-Cr bond, in the rules:

IR-10.2.5.1 The eta (η) convention
(…)
Example 26.

not "…(3 Cr—Cr)".

The purpose of the rule seems to be to just specify (only) the connectivity, i.e. the count of such and such metal-metal connections.

IR-9.2.5.6 Trinuclear and larger structures
(…)
Example 6.

octacarbonyl-1κ4C,24κC-bis(triphenylphosphane-3κP)-triangulo-diironplatinum(Fe—Fe)(2 Fe—Pt)

One could speculate that they, to denote the bond order as well, intend to use a multiplied Em dash character variants, which are non-existent presently.

Another motivation might have been that the bond nature / bond-order is often disputed in organometallics.

• Your name can denote a different isomer (though maybe impossible), with the two ligands oriented the same way, not say antiparallel, or head-tail, but head-head. I.e. one of the ligand flipped, and connected to chromiums the other way.

From the rules:

IR-10.2.5.1 The eta (η) convention
(…)
Example 24.

I.e. repeating the ligands name, with the 1/2 metal locants swapped. Which, however, is obviously inconvenient, maybe the rules will be improved in the future to somehow support relative locants, or so.

• 2,6-{[2,6-di(1-methylethyl)]diphenyl}phenyl part (you construction in points 3, 4) is weird.

The standalone ligand residue would be named according to the organic chemistry nomenclature:

• 2,6-bis[2,6-di(1-methylethyl)phenyl]phenyl, more correctly
• 2,6-bis[2,6-di(propan-2-yl)phenyl]phenyl
• Inorganic nomenclature does not have so strict "preferred name" concept. Table IR-10.1 lists isopropyl as an acceptable alternative name for the propan-2-yl. So I would go with 2,6-diisopropylphenyl instead of 2,6-di(1-methylethyl)phenyl or , to not complicate the name further.

• Problematic locant for the kappa notation in the ligand 2,6-{[2,6-di(1-methylethyl)]diphenyl}phenyl-1κC1,2κC7. Maybe a prime symbol / apostrophe could be used like in biphenyls, terphenyls..

I think it should be rewritten to more verbose, but unambiguous 2-(2,6-diisopropylphenyl-1κC1)-6-(2,6-diisopropylphenyl)phenyl-2κC1 (both locant superscripts after κC's are now 1, but its not a problem).

And one note:

• Even though the rules state

IR-9.2.4.3 Comparison of the eta and kappa conventions

The eta convention (Section IR-10.2.5.1) is applied in cases where contiguous donor atoms within a given ligand are involved in bonding to a central atom. Thus, it is used only when there is more than one ligating atom, and the term η1 is not used.

in the literature(web) for the structure with a more complicated ligand

featuring the similar 2,6-di(iPr)Ph···Cr bond, the following shortcut is used