Imperial College London

Henry Rzepa

Unusual bonds are always intriguing, and the Xe-Xe bond is no exception.
It was first reported
([10.1002/anie.199702731](http://dx.doi.org/10.1002/anie.199702731){target="_blank"})
for the species Xe~2~^+^. Sb~4~F~21~^--^ and its length (3.09Å) was
claimed as "unsurpassed in length in main group chemistry by any other
element -element bond". [Krapp and
Frenking](http://dx.doi.org/10.1002/chem.200700467){target="_blank"}
then creatively tweaked the bond (in a computer). The counterion was
replaced by C~60~, and the two xenon atoms placed inside! Buckyballs
have a fascinating ability to absorb electrons, up to six in fact, from
whatever is placed inside the cavity, and so this assembly acts as a
rather intriguing ion-pair. So the issue reduces to how many electrons
does C~60~ manage to scavenge from two Xenon atoms, and what is the
nature of any resulting bonding formed between these two atoms?

Before taking a look at that question, I note first a [previous
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=580){target="_blank"}
in which I speculate upon the ultimate chemical bond, a 5f-φ bond
between two uranium atoms inside C~60~. The U~2~ atoms have indeed been
stripped of the full complement of six electrons, and the resulting
U~2~^6+^ ion is
[claimed](http://dx.doi.org/10.1021/ja067281g){target="_blank"} to
support a φ bond. But back to Xe. Krapp and Frenking conclude (from NBO
charges) that \~1 electron has been stripped out of Xe~2~, and the
resulting (calculated) Xe-Xe bond length is shortened to 2.49Å, which is
actually less than calculated for an isolated
Xe~2~^[2]{style="font-size: 11px;"}+^ ion (2.75Å). This is an unusual
(and it has to be said hypothetical) example of a compressed
(thermodynamically unstable) bond which would certainly "explode" if
released from its prison, reclaiming the electron in the process. The
system is also of interest given the unusual nature of the (charge
shift) bonding in [F~2~, Cl~2~,
Br~2~](http://www.ch.imperial.ac.uk/rzepa/blog/?p=1722){target="_blank"}
and also what the effect of injecting electrons into an aromatic
periphery of carbon atoms would be (adding two electrons nominally
[subverts](http://dx.doi.org/10.1021/jo801022b){target="_blank"} an
*aromatic* 4n+2 rule into an *antiaromatic* 4n one).

I decided to take another look at Xe~2~@C~60~ because since the original
study of this species in 2007, computational methodology has evolved to
(a) allow the effect of dispersion forces to be included and (b) if the
outer skeleton does indeed absorb electrons, a (continuum) solvation
correction may also [influence the
properties.](http://comporgchem.com/blog/?p=1631#comments){target="_blank"}
This was done as a [ωB97XD/cc-pVDZ/SCRF=water
calculation](http://hdl.handle.net/10042/to-9225){target="_blank"}, with
aug-cc-pVDZ-pp on Xe.

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![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/08/Xe2@c60-charges.jpg "Xe2@c60-charges"){.size-full
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Xe2@C60 showing charge distribution. Click for 3D.
:::

The result is shown with the  atoms colour-coded for charge distribution
(red = -ve, green = +ve), showing the two six-membered rings opposite
the two  Xe atoms have accumulated electrons, at the expense of the Xe
and the central carbon atoms. Appropriately, these two  end rings are
also de-aromatised, with ring bonds of  1.498, 1.424, 1.473, 1.486,
1.473, 1.424Å, compared with 1.451/1.388Å for pure C~60~ with nothing
inside. The results of a QTAIM analysis of the electron density ρ(r)
show a Xe-Xe bond critical point (ν~Xe-Xe~ 401 cm^-1^) with ρ 0.114, and
∇^2^ -0.0013 for a length  of 2.43Å. ELF shows a  Xe-Xe basin
integrating to 0.81 electrons. All of this is pretty much in accord
with [Krapp and
Frenking](http://dx.doi.org/10.1002/chem.200700467){target="_blank"}'s
original very comprehensive study. So we see how the effect of pressure
can induce (1-electron) bonds to form between atoms which would normally
be considered impossible as bonding partners. It is also an unusual
example where ionisation is accompanied by covalent bond formation
(normally, ionization is at the expense of a covalent bond).

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


Unusual bonds are always intriguing, and the Xe-Xe bond is no exception.

Messaggi di Rogue Scholar

Extreme chemical intimacy: the Xe2@C60 ion-pair.