Imperial College London

Henry Rzepa

Members of the chemical FAIR data community have just met in Orlando
(with help from the [NSF, the American National Science
Foundation](https://iupac.org/event/fair-publishing-guidelines-for-spectral-data-and-chemical-structures/){target="_blank"
rel="noopener noreferrer"}) to discuss how such data is progressing in
chemistry. There are a lot of themes converging at the moment. Thus this
article\[cite\]10.1039/c7np00064b\[/cite\] extolls the virtues of having
raw NMR data available in natural product research, to which we added
that such raw data should also be made **FAIR** (Findable, Accessible,
Interoperable and Reusable) by virtue of adding rich metadata and then
properly registering it so that it can be searched. These themes are
combined in another article which made a recent
appearance.\[cite\]10.1021/acsomega.8b03005\[/cite\]

One of the speakers made a very persuasive case based in part on *e.g.*
the following three molecules which are discussed in the first
article\[cite\]10.1039/c7np00064b\[/cite\] (the compound numbers are
taken from there). The question was posed at our meeting: ***why did the
referees not query these structures***? And the answer in part is to
provide referees with access to the full/primary/raw NMR data (which
almost invariably they currently do not have) to help them check on the
peaks, the purity and indeed the assignments. I am sure tools that do
this automatically from such supplied data by machines on a routine
basis do exist in industry (and which is something FAIR is designed to
enable). Perhaps there are open source versions available?

<table data-border="1">
<colgroup>
<col style="width: 33%" />
<col style="width: 33%" />
<col style="width: 33%" />
</colgroup>
<tbody>
<tr class="header">
<th>17</th>
<th>18</th>
<th>19</th>
</tr>
&#10;<tr class="odd">
<td><a
href="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2019/04/17.svg"><img
src="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2019/04/17.svg"
class="aligncenter size-large wp-image-20602" decoding="async"
width="100" /></a><br />
 </td>
<td><a
href="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2019/04/18.svg"><img
src="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2019/04/18.svg"
class="aligncenter size-large wp-image-20603" decoding="async"
width="100" /></a></td>
<td><a
href="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2019/04/19.svg"><img
src="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2019/04/19.svg"
class="aligncenter size-large wp-image-20604" decoding="async"
width="250" /></a></td>
</tr>
<tr class="even">
<td>328[cite]10.1002/ejoc.201301308[/cite]</td>
<td>348</td>
<td>713</td>
</tr>
</tbody>
</table>

Here I suggest a particularly simple and rapid "reality check" which I
occasionally use myself. This is to compute the *steric energy* of the
molecule using molecular mechanics. The mechanics method is basically a
summation of simple terms such as the bond length, bond angle, torsion
angle, a term which models non bonded repulsions, dispersion attractions
and electrostatic contributions. The first three are close to zero for
an unstrained molecule (by definition). The last three terms can be
negative or positive, but unless the molecule is protein sized, they
also do not depart far from zero. A suitable free tool that packages all
this up is [Avogadro](https://avogadro.cc).

The procedure is as follows

1.  Start from the Chemdraw representation of the molecule. If the
    publishing authors have been FAIR, you might be able to acquire that
    from their deposited data. Otherwise, redraw it yourself and save as
    *e.g.* a molfile or Chemdraw .cdxml file.
2.  Drop into Avogadro, which will build a 3D model for you using
    stereochemical information present in the Chemdraw or Molfile.
3.  In the ** E** tool (at the top on the left of the Avogadro menu)
    select *e.g.* the MMFF94 force field. This is a good one to use for
    "organic" molecules for which the total steric energy for "normal"
    molecules is likely to be \< 200 kJ. Calculate that for your system;
    this normally takes less than one minute to complete. The values
    obtained for the three above are shown in the table. All three are
    well over 200 kJ/mol, which should set alarm bells ringing.
4.  A "more reasonable" structure for **17** is shown below. This has a
    steric energy of 152 kJ/mol, some 176 kJ/mol lower than the original
    structure. This does not of itself "prove" this alternative, but it
    is a starting point for showing it might be
    correct.[![](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2019/04/17-correct1.svg){.aligncenter
    .size-large .wp-image-20615 decoding="async"
    width="150"}](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2019/04/17-correct1.svg)Of
    course mis-assigned but otherwise reasonable structures are unlikely
    to be revealed by the steric energy test. But impossible ones will
    probably always be flagged as such using this procedure. 

------------------------------------------------------------------------

**Postscript**: Hot on the heels of writing this, the molecule
**Populusone** came to my
attention.\[cite\]10.1021/acs.orglett.9b00423\[/cite\] On first sight,
it seems to have some of the attributes of an "impossible molecule"
(click on diagram below for 3D coordinates).  
![](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2019/04/liukaixin_384.svg){.aligncenter
.size-full .wp-image-20631 decoding="async"
onclick="jmolApplet([450,450],'load wp-content/uploads/2019/04/Popusulin.mol;spin 3;','c2');"
width="300"}  
However, it has been fully characterised by x-ray analysis! The steric
energy using the method above comes out at 384 kJ/mol, which in the
region of impossibility! This can be decomposed into the following
components: bond stretch 30, bend 51, torsion 32, van der Waals
(including repulsions) 177, electrostatics 87 (+ some minor cross
terms). These are fairly evenly distributed, with internal steric
repulsions clearly the largest contributor. The C=C double bond is
hardly distorted however, which is in its favour. Clearly a natural
product can indeed load up the unfavourable interactions, and this one
must be close to the record of the most intrinsically unstable natural
product known!

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rel="author"}
:::


Members of the chemical FAIR data community have just met in Orlando (with help from the NSF, the American National Science Foundation) to discuss how such data is progressing in chemistry. There are a lot of themes converging at the moment.

Impossible molecules.

[Previously](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10448){target="_blank"},
I looked at the historic origins of the so-called π-complex theory of
metal-alkene complexes. Here I follow this up with some data mining of
the crystal structure database for such structures.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/pi-complex.svg){decoding="async"
width="300"}

Alkene-metal \"π-complexes\" have what might be called a
representational problem; they do not happily fit into the standard
Lewis model of using lines connecting atoms to represent electron pairs.
Structure **1** was the original representation used by
[Dewar](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10448){target="_blank"}
intending the meaning of partial back donation from a filled metal
orbital to the empty π\* of the alkene. At the other extreme these
compounds can be called metallacyclopropanes (**2**) in which only
single bonds feature (these can be thought of as representing full back
bonding from metal to alkene and full forward bonding from alkene to
metal). Representations **3** and **4** are a more fuzzy blend of these,
implying some sort of partial bond order for the metal-carbon bonds.
Taken together, they imply that the formal bond order of the C-C bond
might vary between single to double. Structures **1** and **2** in
particular imply that there might be two distinct ways in arranging the
bonding and that π-complexes and metallacyclopropanes might therefore be
distinct valence-bond isomers, each potentially capable of separate
existence.

Why do these representations matter? Well, I am going to mine the
crystal structure database for these species to try to see if there is
any evidence for a bimodal distribution in the C-C lengths, perhaps
indicating evidence of the isomerism suggested above. Such a structural
database is indexed against atom-pair connectivity in the first instance
and then bond type; one can specify the following types of bond
connecting any two atoms: *single*, *double*, *triple*, *quadruple*,
*polymeric*, *delocalised*, *pi* and *any*. It is not entirely obvious
which if any of these types apply to structure **1** (it is not possible
to draw a bond ending at the mid-point of another bond using the
Conquest structure editor); the dashed lines in structures **3** and
**4** could be classed as *delocalised*, *pi*, or most generally *any*.
The search query can be constructed thus, where the two carbons carry R
which can be either H or C and all four C-R bonds are specified as
acyclic (to try to avoid complications by excluding compounds such as
cyclic metallacenes). Because representation **1** cannot be constructed
in the editor, I am going to specify that each carbon carries four bonds
of any type in the first instance. The torsion specified is defined as
R-C-C-M and the full queries can be found deposited
here.\[cite\]10.14469/hpc/642\[/cite\]

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/zeise-sq.jpg){decoding="async"
width="300"}

If the metallacyclopropane representation **2** is defined with explicit
single bonds, one gets only 22 hits (no errors, no disorder, R \< 0.1).
The distribution of C-C bond lengths is shown below. Already one sees a
representational problem emerging. A true metallacyclopropane might be
expected to show a C-C single bond length, say \> \~1.5Å. But only one
or two of these examples actually have this value, the most probable
value being \~1.4Å.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/Z-cc-single.jpg){decoding="async"
width="440"}

Using representation **3**, one gets 1861 hits, but as before one sees a
maximum at \~1.4Å with a tail reaching to both single and double bond
values for the C-C distance.^‡^

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/Z-cc-double.jpg){decoding="async"
width="440"}

If the C-C bond is also specified as \"any\", the hits increase to 3948,
but the bond length distribution is still very similar, with no sign of
any bimodal distribution.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/Z-all.jpg){decoding="async"
width="440"}

Such a distribution is however found if the torsions between the R-C
bond vector and the C-M bond vector are plotted (for *all* types of
bond). A large number of the complexes have a torsion \<90°, which
suggests that in fact the substituent R is probably interacting with the
metal (even though this would lead to formal cyclicity, specifying R-C
as acyclic does not detect this interaction). Could this be masking a
bimodal distribution in the C-C lengths?

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/pi-complex-torsions.jpg){decoding="async"
width="440"}

If the previous search is repeated, but this time specifying that all
four torsions must lie in the range 90-180° (the range expected for a
\"classical\" alkene-metal complex and selecting only the top right hand
side cluster in the plot above) the reduced value of 1051 hits are
obtained, but the monomodal distribution remains.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/Z-cc-dist-4torsion-120.jpg){decoding="async"
width="440"}

For this last set, here is a plot of the two C-metal bond length, with
colour indicating the C-C bond length, indicating the two C-metal bonds
are clearly linearly correlated.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/Z-cc-dist.jpg){decoding="async"
width="440"}

One final variation;  the atom on either C can only be H or a
4-coordinate (sp^3^) carbon; 645 hits. Again, a monomodal distribution
centered at 1.4Å.

So this foray through metal alkene complexes suggests that there is a
continuum between the formal metallacyclopropane with a C-C single
bond and the only slightly perturbed alkene-metal complex with a C=C
double bond. Whilst this would not prevent any one of these compounds
existing as two distinctly different valence-bond isomers, it makes it
very unlikely. I had noted in an [earlier
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=15635){target="_blank"}
that for molecules of the type RX≡XR (X=Si, Ge, Sn, Pb) that there was
indeed a clear bimodal distribution of the X-X lengths evident in the
crystal structures (for a relatively small sample number). The
structures **1-4** shown at the start of this post are all simply just
variations in a continuum and not distinct isomers.

**POSTSCRIPT:**  I noted above the bimodel distribution in compounds
involving formal triple bonds. So I repeated the search above for
π-complex metal-[**alkyne**]{style="color:#FF0000;"} complexes.
Specifying an acyclic C-R bond, and *any* for the CC bond type, one gets
the following.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/c-triple-c.jpg){decoding="async"
width="440"}

There is now a tantalizing suggestion of two clusters, one at 1.3 and
another at 1.4Å. The torsional distribution shows that the latter
distance appears to be associated with much smaller torsions, whereas
the top right cluster is associated with shorter lengths.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/c-triple-c-torsions.jpg){decoding="async"
width="440"}

If the torsions are restricted to the range 90-180, then the histogram
looses the smaller cluster, and perhaps gains a second cluster at 1.22Å?
 As I said, all quite tantalizing!

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/c-triple-c-90-180.jpg){decoding="async"
width="440"}

------------------------------------------------------------------------

^‡^The tail in all the histograms extends into the 1.1-1.3Å region,
which seems unreasonable for a carbon where four bonds are specified.
This region probably represents errors in the crystallographic analysis
or reporting. But who knows, perhaps some very unusual compounds are
lurking there!

::: {.citizen-ex__pane style=""}
 
:::

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rel="author"}
:::


Previously, I looked at the historic origins of the so-called π-complex theory of metal-alkene complexes. Here I follow this up with some data mining of the crystal structure database for such structures. Alkene-metal "π-complexes" have what might be called a representational problem; they do not happily fit into the standard Lewis model of using lines connecting atoms to represent electron pairs.

A wider look at π-complex metal-alkene (and alkyne) compounds.

The phenomenon of *bond stretch isomerism*, two isomers of a compound
differing predominantly in just one bond length, is one of those
chemical concepts that wax and
occasionally wane.\[cite\]10.1016/S1631-0748(02)01380-2\[/cite\] Here I
explore such isomerism for the elements Ge, Sn and Pb.

In [one earlier
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=14043){target="_blank"
rel="noopener"}, I noted a form of bond stretch isomerism that can arise
from a Jahn-Teller distortion ending in two different geometries in
which one or more pairs of bonds swap short/long lengths. Examples
include substituted
cyclo-octatetraenes\[cite\]10.1039/P29920001951\[/cite\] and octahedral
d^9^-Cu(II) complexes.\[cite\]10.1021/ja905399x\[/cite\] A more
interesting seminal possibility was implied by [G. N.
Lewis](http://www.rsc.org/events/detail/21281/the-atom-and-the-molecule-a-symposium-celebrating-gilbert-n-lewis){target="_blank"
rel="noopener"} a century ago when discussing the arrangement of
electrons in a (carbon-carbon) triple
bond.\[cite\]10.1021/ja02261a002\[/cite\]

![lewis1](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/lewis1-1-1024x421.jpg){.aligncenter
.size-large .wp-image-15802 loading="lazy" decoding="async" height="185"
width="450"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/lewis1-1-1024x421.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/lewis1-1-300x123.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/lewis1-1-768x316.jpg 768w"
sizes="(max-width: 450px) 100vw, 450px"}  
[\*]{style="color:#FF0000;"}It took \~50 years to prove this assertion
wrong.\[cite\]10.1021/ic50025a016\[/cite\]

[In a
commentary](http://www.ch.imperial.ac.uk/rzepa/blog/?p=14037){target="_blank"
rel="noopener"}, I reported the results of a search of the crystal
structure database for the geometries associated with RX≡XR systems (X=
C, Si, Ge, Sn, Pb). Here I focus the
search\[cite\]10.14469/hpc/249\[/cite\] specifically for X=Sn,Ge; this
version of bond stretch isomerism also allows angles to change (=
rehybridisation at atoms) in order to provide a mechanism for a barrier
separating the two forms.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/Sn-Sn.jpg "Cr"){.aligncenter
.size-full .wp-image-15761 decoding="async" width="400"}

For X=Sn, note the presence of up to three clusters, although the
relatively low number of hits makes the statistics less certain.

1.  The hotspot cluster centered around angles of 125° and a Sn-Sn
    distance of \~2.6Å.
2.  Another with angles of \<100° and Sn-Sn distances of \~3.3Å.
3.  A third with angles of \<100° and Sn-Sn distances of 2.8Å, which may
    or may not be a genuine unique form of bonding.

This pattern was commented on in 2010
by Power\[cite\]10.1039/C0SC00240B\[/cite\], whose group
synthesized most of the examples in the hits above.^‡^ A plot of
compounds with Ge-Ge bonds reveals both similarity with (two, possibly
three clusters) and difference from (the clusters are closely spaced in
terms of the Ge-Ge bond length, but separated in terms of angle) Sn.^†^

![GeGe](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/GeGe.jpg){.aligncenter
.size-full .wp-image-15761 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/GeGe.jpg 653w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/GeGe-300x223.jpg 300w"
sizes="(max-width: 653px) 100vw, 653px"}

Time for some computations (which at least will remove random errors in
the geometry). I selected the only known example of an
RPb-PbR compound\[cite\]10.1021/ja993346m\[/cite\] as a seed and put it
through a B3LYP+D3/Def2-TZVPP calculation (with 172 atoms and 2920 basis
functions, this is a relatively large calculation!), which reproduces
the known structure pretty well (table).

[![QIMQUY](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/QIMQUY.svg){.aligncenter
.size-large .wp-image-15675 decoding="async"
width="300"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/QIMQUY.svg){rel="attachment wp-att-15675"}

So what about another bond stretch isomers? The Pb=Pb variation is
indeed a stable minimum around 28.0 kcal/mol above the known structure,
which seems to put this form out of experimental reach (with this
ligand/aryl group at least). With Sn for the same aryl ligand, the
energy difference is smaller (\~15.8 kcal/mol for this ligand; Powers
reports other systems where the energy difference may be only \~5
kcal/mol). Judging by the distribution of the 13 hits recovered from the
CSD search, both bond stretch isomers may be accessible
experimentally. The calculations show that the GeGe bond isomers are
much closer in energy than SnSn (for this ligand). For all three metals
however, the calculated difference in the metal-metal length for the two
isomers is \~0.45 -- 0.52Å. This strongly suggests that whereas the SnSn
plot above is demonstrating bond length isomerism, the GeGe plot may not
be; at least not of the same type that the calculations here are
revealing (*via* the Wiberg bond orders).

|        |                 |                                                   |                                                 |                   |                                     |
|--------|-----------------|---------------------------------------------------|-------------------------------------------------|-------------------|-------------------------------------|
| System | Relative energy | XX distance                                       | RXX angle                                       | Wiberg bond order | DataDOI                             |
| Pb=Pb  | +28.0           | 2.767                                             | 118.7                                           | 1.666             | \[cite\]10.14469/ch/191856\[/cite\] |
| Pb-Pb  | 0.0             | 3.215 (3.188)^\[cite\]10.1021/ja993346m\[/cite\]^ | 93.7 (94.3)^\[cite\]10.1021/ja993346m\[/cite\]^ | 0.889             | \[cite\]10.14469/ch/191873\[/cite\] |
| Sn=Sn  | +15.8           | 2.640                                             | 123.1                                           | 1.911             | \[cite\]10.14469/ch/191884\[/cite\] |
| Sn-Sn  | 0.0             | 3.126                                             | 95.5                                            | 0.892             | \[cite\]10.14469/ch/191881\[/cite\] |
| Ge=Ge  | +0.5            | 2.263                                             | 125.2                                           | 2.138             | \[cite\]10.14469/ch/191882\[/cite\] |
| Ge-Ge  | 0.0             | 2.777                                             | 99.7                                            | 0.866             | \[cite\]10.14469/ch/191883\[/cite\] |

No doubt the particular bond length form is being facilitated by the
nature of the ligand and the steric interactions therein imparted, both
repulsive AND attractive. These interactions can be visualised *via* NCI
(non-covalent-interaction) plots (click on the image to obtain a
rotatable 3D model). First Pb-Pb followed by Pb=Pb. Note how in both
cases, the PbPb region is enclosed in regions of weak attractive
dispersion interactions, which however avoid the \"hemidirected\" inert
Pb lone pairs.\[cite\]10.1039/C4DT01406E\[/cite\]

![Pb-Pb](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/pb-pb-nci.jpg){.aligncenter
.size-large .wp-image-15675 decoding="async"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2016/02/Pb-Pb_den.cub.xyz;isosurface colour red blue wp-content/uploads/2016/02/Pb-Pb_den.cub.jvxl opaque;');"
width="400"}
![Pb=Pb](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/02/pb=pb-nci.jpg){.aligncenter
.size-large .wp-image-15675 decoding="async"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2016/02/Pb=Pb_den.cub.xyz;isosurface colour red blue wp-content/uploads/2016/02/Pb=Pb_den.cub.jvxl opaque;');"
width="400"}

So in the end we have something of a mystery. There is evidence from
crystal structures that at least two bond-stretch isomers of RSnSnR
compounds can form, but the calculations indicate that the Sn=Sn form is
significantly higher in energy (although not impossibly so for thermal
accessibility). Conversely, the Ge=Ge equivalent is very similar in
energy to a Ge-Ge form with a significantly longer bond length, but
there seems no crystallographic evidence for such a big difference in
bond lengths. Perhaps the answer lies with the ligands?

It seems particularly appropriate on the centenary of G. N. Lewis\'
famous paper in which he clearly notes the possibility of three isomeric
forms for the *triple bond*, to pay tribute to the impact his
suggestions continue to make to chemistry.

------------------------------------------------------------------------

^‡^The individual entries can be inspected via the following
dois: \[cite\]10.5517/CCYYS5T\[/cite\],\[cite\]10.5517/CC6942P\[/cite\],\[cite\]10.5517/CCTWKLT\[/cite\],\[cite\]10.5517/CCTWKMV\[/cite\],\[cite\]10.5517/CCTWKPX\[/cite\],\[cite\]10.5517/CCTWKQY\[/cite\],\[cite\]10.5517/CCTWKRZ\[/cite\],\[cite\]10.5517/CCTWKT1\[/cite\],\[cite\]10.5517/CCNYK04\[/cite\],\[cite\]10.5517/CC7FYSC\[/cite\]

^†^You can view individual entries via the following DOIs:
\[cite\]10.5517/CC13R2MK\[/cite\],\[cite\]10.5517/CCT4VVM\[/cite\],\[cite\]10.5517/CC136VY3\[/cite\],\[cite\]10.5517/CC9ZXBH\[/cite\],\[cite\]10.5517/CC61ZRY\[/cite\],\[cite\]10.5517/CCTWKS0\[/cite\],\[cite\]10.5517/CCTWKNW\[/cite\],\[cite\]10.5517/CCTWKKS\[/cite\],\[cite\]10.5517/CCYYS4S\[/cite\],\[cite\]10.5517/CCYYS3R\[/cite\]

------------------------------------------------------------------------

#### Acknowledgments

This post has been cross-posted in PDF format at
[Authorea](https://doi.org/10.15200/winn.145586.67442){rel="noopener"
target="_blank"}.

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rel="author"}
:::


The phenomenon of
<em>
 bond stretch isomerism
</em>
, two isomers of a compound differing predominantly in just one bond length, is one of those chemical concepts that wax and occasionally wane.[cite]10.1016/S1631-0748(02)01380-2[/cite] Here I explore such isomerism for the elements Ge, Sn and Pb.  In one earlier post, I noted a form of bond stretch isomerism that can arise from a Jahn-Teller distortion ending in two different geometries

Messaggi di Rogue Scholar

Impossible molecules.

A wider look at π-complex metal-alkene (and alkyne) compounds.

Bond stretch isomerism. Did this idea first surface 100 years ago?