Imperial College London

Henry Rzepa

C~2~ (dicarbon) is certainly
[interesting](http://www.ch.imperial.ac.uk/rzepa/blog/?p=3065 "Following one’s nose: a quadruple bond to carbon. Surely I must be joking!"){target="_blank"}
from a theoretical point of view. Whether or not it can be described as
having a *quadruple bond* has induced much passionate
discussion\[cite\]10.1038/nchem.1263\[/cite\],\[cite\]10.1002/anie.201208206\[/cite\],\[cite\]10.1002/anie.201301485\[/cite\],\[cite\]10.1002/anie.201302350\[/cite\].
Its occurrence in space and in flames is also well-known. But does it
have what might be called a conventional chemistry? Other highly
reactive species
([cyclobutadiene](http://www.ch.imperial.ac.uk/rzepa/blog/?p=9894 "To be cyclobutadiene, or not to be, that is the question?  You decide."){target="_blank"}
is a well-known example) can often be tamed by
[trapping](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10498 "Au and Pt π-complexes of cyclobutadiene."){target="_blank"}
as a ligand coordinated to a metal and so one might speculate upon how
C~2~ responds to the proximity of a metal. As is noted
here\[cite\]10.1002/anie.201208206\[/cite\], dicarbon as a ligand has
been known a long time as part of what is referred to as carbide
chemistry. In this regard it is thought of as the di-anion, C~2~^2-^
(and isoelectronic therefore with dinitrogen). Thus calcium carbide, but
in fact the degree to which the dicarbon can absorb electrons is thought
to be wide (as judged by the resulting C-C bond length,
see\[cite\]10.1002/anie.201208206\[/cite\]). Here I take a look at just
one metal carbide\[cite\]10.1016/j.jssc.2008.08.005\[/cite\] that caught
my eye (there are hundreds of others, many no doubt equally
interesting!).

![dicarbon](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/07/dicarbon1.svg){.aligncenter
.size-full .wp-image-10783 decoding="async"}

One first notices that the standard attempt at a representation above
does not do it justice; a Lewis valence bond drawing (in which electrons
are accounted by partitioning into bonds) it most certainly is not. Nine
carbons and four osmiums have absorbed 21 electrons, donated (formally)
from seven lanthanum atoms. But easily discerned is that of the nine
carbons, four are dicarbon and five are mono carbon. It is worth
exploring the immediate environment of each of these types. The
monocarbon is in fact
[hexa-coordinate](http://www.ch.imperial.ac.uk/rzepa/blog/?p=2469 "Solid carbon dioxide: hexacoordinate carbon?"){target="_blank"}d
by two Os and four La, a form of carbon coordination that was only
relatively recently identified.

::: {#attachment_10789 .wp-caption .aligncenter style="width: 390px"}
![Click for
3D](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/07/LaOsC1.jpeg){.wp-image-10789
loading="lazy" decoding="async"
aria-describedby="caption-attachment-10789"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/07/XOKKEO-C1.cif;');"
width="380" height="303"}

Click for 3D
:::

But the dicarbon unit is if anything even stranger. The C-C bond length
is \~1.316Å (a relatively long distance compared to many carbides) more
or less commensurate with a double bond. The carbons are both end-on
σ-coordinated and also triply π-coordinated. with one more metal
coordinating in a manner somewhat in-between these two modes. One carbon
is 5-coordinate, the other 6-coordinate. The Os-C bond length has a
relatively short value of 1.93Å (the sum of the double bond covalent
radii for Os and C is 1.83, the single bond radii 2.04), and so it might
be tempting to represent it as Os=C=C.

::: {#attachment_10790 .wp-caption .aligncenter style="width: 459px"}
![Click for
3D](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/07/LaOsC21.jpeg){.wp-image-10790
loading="lazy" decoding="async"
aria-describedby="caption-attachment-10790"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/07/XOKKEO-C2.cif;');"
width="449" height="346"}

Click for 3D
:::

This one example reminds us that even an element such as carbon, where
one might imagine the bonding environments would be well-known, can
still reveal unusual behaviours. Taking a look at the histogram below,
which indicates C-C lengths in Metal-CC-Metal complexes (where the
carbon coordination is restricted to 2) indicates the diversity of
behaviour possible with this simple little ligand.

![LaOsCC-hist](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/07/LaOsCC-hist.jpeg){.aligncenter
.wp-image-10795 loading="lazy" decoding="async" width="505"
height="243"}

::: {.wp_orcid_field}
[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
:::


C
<sub>
 2
</sub>
(dicarbon) is certainly interesting from a theoretical point of view. Whether or not it can be described as having a
<em>
 quadruple bond
</em>
has induced much passionate discussion[cite]10.1038/nchem.1263[/cite],[cite]10.1002/anie.201208206[/cite],[cite]10.1002/anie.201301485[/cite],[cite]10.1002/anie.201302350[/cite]. Its occurrence in space and in flames is also well-known.

Messaggi di Rogue Scholar

Is dicarbon (C2) a molecule of chemical interest?