Imperial College London

Henry Rzepa

Recollect [the
suggestion](http://www.ch.imperial.ac.uk/rzepa/blog/?p=19251) that
diazomethane has hypervalent
character\[cite\]10.1039/C5SC02076J\[/cite\]. When I looked into this, I
came to the conclusion that it probably was mildly hypervalent, but on
carbon and not nitrogen. Here I try some variations with substituents to
see what light if any this casts.  
[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/diazomethane2.svg){.aligncenter
.size-large .wp-image-19309 decoding="async"
width="450"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/diazomethane2.svg)

I have expanded the resonance forms of diazomethane by one structure
from those shown in the previous two posts (a form by the way not
considered in the original article\[cite\]10.1039/C5SC02076J\[/cite\])
to include a nitrene. This takes us back to an [earlier
suggestion](http://www.ch.imperial.ac.uk/rzepa/blog/?p=19133) on this
blog that HC≡S≡CH is not a stable species but a higher order saddle
point which distorts down to a *bis*-carbene, together with the
suggestion that hypervalent triple bonds have the option of converting
four of the six electrons into two carbene lone pairs, replacing the
triple bond with a single bond. This in turn harks back to [G. N.
Lewis'](http://www.ch.imperial.ac.uk/rzepa/blog/?p=14037) 101 year old
idea for acetylene itself!

To explore this mode, I start by replacing the terminal ≡N in
diazomethane with a ≡C-Me group, which cannot absorb electrons into
lone-pairs in the manner that nitrogen can. A ωB97XD/Def2-TZVPP
calculation^‡^ reveals that the linear form is a transition state for
interconversion into a carbene. The IRC for the process (below) shows
this carbene is \~10 kcal/mol lower than the linear "hypervalent" form. 

[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/E-1.svg){.aligncenter
.size-large .wp-image-19320 decoding="async"
width="450"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/E-1.svg)

NBO analysis of this transition state reveals a similar orbital pattern
to diazomethane itself, including a non-bonding orbital on the H~2~C
carbon. The Wiberg carbon bond indices are 3.6764 and N 3.6454 and the
bond orders C=N 1.1390 and N=CMe 1.6192.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe.jpg){.aligncenter
.size-large .wp-image-19319 decoding="async" width="350"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe.jpg 689w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe-300x224.jpg 300w"
sizes="(max-width: 689px) 100vw, 689px"}

ELF analysis of this transition state reveals the presence of two
non-bonding pairs on the carbon atoms either side of the nitrogen but
unshared with it, with populations of 1.19e and 1.37e (DFT). That
nitrogen really does not like excess electrons! The four atoms C,N,C,C
have ELF valence basins totalling 8.00, 6.94, 7.69 and 7.92e (DFT) or
8.07, 7.07 and 7.61e (CASSCF), suggesting that unlike diazomethane
itself, the octet-excess induced hypervalence on carbon is slightly
decreased.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe_ELF-1-1024x382.jpg){.aligncenter
.size-large .wp-image-19318 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe_ELF-1-1024x382.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe_ELF-1-300x112.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe_ELF-1-768x287.jpg 768w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe_ELF-1.jpg 2025w"
sizes="(max-width: 1024px) 100vw, 1024px"}

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe_CAS_ELF-1024x612.jpg){.aligncenter
.size-large .wp-image-19323 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe_CAS_ELF-1024x612.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe_CAS_ELF-300x179.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCMe_CAS_ELF-768x459.jpg 768w"
sizes="(max-width: 1024px) 100vw, 1024px"}

Pumping even more electrons in by replacing the ≡C-Me group with
≡C-NH~2~ does not increase any hypervalence, but does induce more
electrons to reside in "lone pairs". Of the four atoms along the chain,
three have "lone pairs" associated with them, a total of 4.83e that do
not contribute to bonds (valence).

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCNH2_ELF-1024x787.jpg){.aligncenter
.size-large .wp-image-19326 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCNH2_ELF-1024x787.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCNH2_ELF-300x230.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCNH2_ELF-768x590.jpg 768w"
sizes="(max-width: 1024px) 100vw, 1024px"}

An electron withdrawing ≡C-CN group replacing the ≡C-NH~2~ reverses the
effect of the latter, but this linear species is still a transition
state for carbon isomerisation:

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCCN_ELF-1024x258.jpg){.aligncenter
.size-large .wp-image-19328 loading="lazy" decoding="async" width="450"
height="113"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCCN_ELF-1024x258.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCCN_ELF-300x76.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/H2CNCCN_ELF-768x193.jpg 768w"
sizes="(max-width: 450px) 100vw, 450px"}

Finally, combining all we have learnt by adding in nitro groups on the
first carbon. This is no longer a transition state but now a stable
species; the sum of the ELF basin integrations around the carbon on the
left reaches 8.95e, slightly higher than the dinitro-diazomethane
discussed in the previous post. The numerical Wiberg atom bond indices
are C 3.8713, N 3.6898, C 3.8503, C 3.9958 and N 3.0288 for the atoms
along the chain, with the first nitrogen the
"least-valent".![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/O2N2CNCCN_ELF-1024x637.jpg){.aligncenter
.size-large .wp-image-19329 loading="lazy" decoding="async" width="450"
height="280"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/O2N2CNCCN_ELF-1024x637.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/O2N2CNCCN_ELF-300x187.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/12/O2N2CNCCN_ELF-768x478.jpg 768w"
sizes="(max-width: 450px) 100vw, 450px"}

So we see that "hypervalence", or at least "octet-excess", which is not
exactly the same as hypervalence since it includes contributions from
non-bonding electrons, is balanced on a knife-edge. Trying to increase
the octet-excess by pumping electrons in turns the system into a
transition state for carbene formation. Octet-excess is seen as a
metastable property, to be relieved by geometric distortions where
possible or localization of electrons into non-bonding lone pairs. And I
remind yet again that no evidence has manifested in calculations of the
molecules above that the central nitrogen of these diazomethane-like
systems has any propensity for octet or valence-excess as implied by the
formula C=N≡X.\[cite\]10.1039/C5SC02076J\[/cite\]

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

^‡^FAIR data for all calculations is available at
DOI: [10.14469/hpc/3476](https://doi.org/10.14469/hpc/3476)

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Recollect the suggestion that diazomethane has hypervalent character[cite]10.1039/C5SC02076J[/cite]. When I looked into this, I came to the conclusion that it probably was mildly hypervalent, but on carbon and not nitrogen. Here I try some variations with substituents to see what light if any this casts.

Messaggi di Rogue Scholar

Are diazomethanes hypervalent molecules? An attempt into more insight by more “tuning” with substituents.