Imperial College London

Henry Rzepa

The concept of a shared electron bond and its property of an order is
almost 100 years old in modern form, when G. N. Lewis suggested a model
for single and double bonds that involved sharing either 2 or 4
electrons between a pair of atoms\[cite\]10.1021/ja02261a002\[/cite\].
We tend to think of such (even electron) bonds in terms of their formal
bond order (an integer), recognising that the actual bond order (however
defined) may not fulfil this value. I thought I would very (very)
briefly review the history of such bonds.

1.  **1916:** G. N. Lewis\[cite\]10.1021/ja02261a002\[/cite\] proposed a
    model for carbon involving a cube with one electron at each corner,
    thus making an octet.^‡^ A **single bond** would be created by two
    atoms sharing a common edge (= 2 shared electrons), and a **double
    bond** by sharing a common face (= 4 shared electrons). The
    recognition that the formal bond order of two could be partitioned
    into one electron pair of σ symmetry and one of π was not achieved
    until \~1929 (by Hückel). It is also now recognised that whilst most
    bonds of order 1 are of type σ, a rare few can be π (these are
    called homo or "suspended" bonds).

2.  **1916:** Lewis also speculates about a rather less well-known model
    comprising "*eight electrons in which pairs are symmetrically placed
    about a center gives ... the model of the tetrahedral carbon atom*."
    He then points out that two tetrahedra, attached by one, two or
    three corners each would represent the single, the double and the
    **triple bond**. The latter "*represents the highest possible degree
    of union between two atoms*". He chooses acetylene as an example,
    representing it as H:C:::C:H and two "tautomers" (we would now call
    them valence bond isomers)^†^ with lower bond orders, these being
    what we now call a bis-carbene and a
    biradical:![Lewis](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/03/Lewis.svg){.aligncenter
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3.  **1965:** It took a remarkable wait of 49 years (a span which
    encompasses the development and maturity of quantum mechanics) to
    extend the "highest possible degree of union" to the quadruple bond,
    identified by Cotton in the previously known compound
    \[Re~2~Cl~8~\]^2-^.\[cite\]10.1021/ic50025a016\[/cite\].  

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    In fact, Mulliken had drawn a quadruple bond between the two carbons
    in C~2~ back in 1939\[cite\]10.1103/PhysRev.56.778\[/cite\] (see
    Table 1, p 779) but he probably thought of it as a very high energy
    excited state and that it did not merit further discussion. The
    latest thoughts are that C~2~  does indeed have (a weak) fourth
    bond\[cite\]10.1002/anie.201208206\[/cite\] in its ground electronic
    state.

4.  **2005:** Another 40 years elapsed before quintuple or "fivefold"
    bonding was discovered by
    Power\[cite\]10.1126/science.1116789\[/cite\] in ArCrCrAr. There has
    been a bit of a race since to discover the shortest example of this
    genre.  

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5.  **2013:** Unlike the lower bond orders, where direct structural data
    for larger molecules is available, speculation about sextuple bonds
    is limited largely to theoreticians, who have been at it for quite a
    while. The latest thinking is summarised
    here\[cite\]10.1039/C2CP43559D\[/cite\] (also doi:
    [10.1039/C2CP43559D](http://dx.doi.org/10.1039/C2CP43559D){target="_blank"}). The
    current best candidates for a sextuple bond include Mo~2~ and W~2~.

6.  What is the limit of the formal integer bond order? I do not believe
    anyone thinks that septuple or octuple bonds (formal or otherwise)
    will be discovered (or even speculated upon) any time soon, but
    there is no fundamental law which would prohibit
    them.\[cite\]10.1038/446276a\[/cite\] Quite possibly if we get
    beyond element 120 in the periodic table, examples might emerge!

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^‡^A formula for predicting the filled electron shells is 2(N+1)^2^,
which gives the values 2, 8, 18,
32\[cite\]10.1126/science.54.1386.59\[/cite\],\[cite\]10.1002/anie.200604198\[/cite\]
50. It is also, as it happens, the rule for 3D aromaticity in clusters.

^†^A bis-carbene form, whilst not appropriate for carbon, may indeed
become more realistic as one proceeds down column 14 of the periodic
table. Thus \[cite\]10.1021/ja0257164\[/cite\], where Ar-Sn≡Sn-Ar has a
C-Sn-Sn bond angle of 125°.

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Or perhaps an even better example\[cite\]10.1039/c0sc00240b\[/cite\]
with a C-Sn-Sn angle of 98°. There is also an example of
C-Pb-Pb\[cite\]10.1021/ja993346m\[/cite\] with an angle of 94°.

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The concept of a shared electron bond and its property of an order is almost 100 years old in modern form, when G. N. Lewis suggested a model for single and double bonds that involved sharing either 2 or 4 electrons between a pair of atoms[cite]10.1021/ja02261a002[/cite]. We tend to think of such (even electron) bonds in terms of their formal bond order (an integer), recognising that the actual bond order (however defined) may not fulfil this

Messaggi di Rogue Scholar

A (very) short history of shared-electron bonds.