CCL: Metal-Carbene sigma type bond

 Sent to CCL by: "Sergio  Manzetti" [sergio.manzetti/./]
 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.
 Best wishes
 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 -
 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 wrote:
     Sent to CCL by: "Sergio  Manzetti" []
     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 ..
         To: sergio.manzetti ..
         Subject: CCL: Metal-Carbene sigma type bond
         Date: Thu, 18 Sep 2014 13:46:58 +0200
         Sent to CCL by: Fedor Goumans [goumans^-^]
         Hi Sergio,
         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
         ('early TMs').
         Best wishes,
         On 9/18/2014 10:44 AM, Sergio Manzetti sergio.manzetti a
             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
         Business Developer
         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
         E-mail: goumans**
         http://www.scm.comthe strange characters on the top line to
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 Tom Albright
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