CCL: Metal-Carbene sigma type bond
- From: "Sergio Manzetti"
- Subject: CCL: Metal-Carbene sigma type bond
- Date: Thu, 25 Sep 2014 03:13:16 -0400
Sent to CCL by: "Sergio Manzetti" [sergio.manzetti/./outlook.com]
Dear Tom. You mention the difference between the d-orbitals versus the p atomic
orbital which makes the difference. Is this simple the HOMO-LUMO gap in few
words, or would it be more intricate, such as the difference between occupied
and non-occupied AOs of both the metal and the carbon. I Had the impression from
one of the books that the antibonding orbital (2py empty) of the carbon forms a
pi bond with an occupied d sub-orbital, assuming dxy (I cannot remember which).
Nevertheless, this interaction is completely gone in the Schrock mechanism, and
it seems that the Schrock is dependent on triplet states, which in turn depend
on the nuclear properties, thus the atom. However, I am still not clear on where
the threshold goes between Fischer and Schrock types, and you mention correctly
that there is a continuum with two extremes on each side. Would this mean that
the Lantanides are more Schrock types, while the 4th row metals are Fischer
I haven't read you book yet, I will get there soon.
Dear Sergio - There are many ways that have been used to describe transition
metal - carbene bonding. The bottom line is whether or not one gets anymore
physical insight to "real" molecules. You are not correct in stating
that Schrock-type complexes are more likely for heavier transition metals. Foe
example, there are many Pd or W complexes which are the Fisher type, i.e. have
carbene carbons that are electrophilic in nature. I would prefer to view these
two types as opposites on a continuum where it is the energetic difference in
the metal d orbitals versus the carbene p AO forming the pi bond which makes the
difference. This is all spelled out fairly clearly (I think) in the Albright,
Burdett, Whangbo "Orbital Interactions in Chemistry" 2nd Ed.book -
complete with plots of the MOs. I am on vacation so I can't give you the page
On Sep 18, 2014, at 9:41 AM, Sergio Manzetti sergio.manzetti%%outlook.com wrote:
Sent to CCL by: "Sergio Manzetti" [sergio.manzetti-.-outlook.com]
Dear Fedor, thanks. I have looked further in the book, and it seems there
are two mechanisms at hand: one Fischer mechanism and one other the Schrock
The difference between the two, is that one considers the Metal bonding
d-electrons as a singlet, while the other as a triplet. The singlet gives
apparently a "regular" double bond by 4 electrons, pairing with the
carbons 2p2. While the other, has one electron in the dxy and the other in the
dxz, each of these unpaired electrons form a pair with the unpaired electrons of
equally considered carbene in its triplet state. Eventually, it says that the
latter (The Schrock mechanism) is more for likely for the heavier transition
metals, while the Fischer more likely for light metals, such as Cr and V.
Your guess on the non-participating 6s2 was right. I take its because the 6s
really has a slightly lower energy than the 5d, at the end of the day (aufbau).
Thanks for your tip. I May use software to check it out.
From: owner-chemistry .. ccl.net
To: sergio.manzetti .. gmx.com
Subject: CCL: Metal-Carbene sigma type bond
Date: Thu, 18 Sep 2014 13:46:58 +0200
Sent to CCL by: Fedor Goumans [goumans^-^scm.com]
You may consider using any of the chemical bonding analysis tools out
there to dig in to the bonding situation (aside from the
phenomenological Dewar-Chatt-Duncanson model). Or if anything, just look
at the Mulliken population analysis.
My guess is that for tantalum carbene complexes the 6s orbitals will not
be occupied at all.
I suggest the thorough paper by Frenking et al. (Chemical bonding in
transition metal carbene complexes, J. Organomet. Chem. 690, p.
6178-6204 (2005)) as a starting point. Most of it is on Cr carbenes, but
you can exctend the analysis to group-5 transition metals as well
On 9/18/2014 10:44 AM, Sergio Manzetti sergio.manzetti a outlook.com
Sent to CCL by: "Sergio Manzetti"
Dear all, I have a question regarding the formation of a sigma-type
bond between Tantalum and the carbene ion. The sigma type bond is apparently a
sp2 hybridized bond. Accounting for the configuration of carbon, it donates its
two 2px electrons which interact with one of the tantalum 6s? The pi type bond
is more clear, it is formed by an empty 2py of the carbon interacting with one
of the 5d electrons. However, if the sigma type bond is formed by two 2px
electrons, where does the second 6s electron go?
Alternatively, this was wrong assumption.
Orginally, I was looking for the energy diagram of the sigma type
bond of the Ta=CH2, but could not find it.
Dr. T. P. M. (Fedor) Goumans
Scientific Computing & Modelling NV (SCM)
Vrije Universiteit, FEW, Theoretical Chemistry
De Boelelaan 1083
1081 HV Amsterdam, The Netherlands
T +31 20 598 7625
F +31 20 598 7629
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