CCL: Metal-Carbene sigma



 Sent to CCL by: "Sergio  Manzetti" [sergio.manzetti~!~outlook.com]
 Dear Tom, thanks, this seemed clear.
 However, I Have an additional point to add. I think the tendencies of 6-row
 transition metals (Lantanides) to subdue to a Scrock mechanism is potentially in
 that their 5d orbitals, inversely to the 3d of lower weight transition metals,
 have much higher energy and favorize much more Hunds' rule, in that the
 electrons rather occupy two single orbitals, rather than pair in a common one.
 This may give a clearer idea to the entire set of mechanisms, however, I take it
 has to be proven first?
 All the best
 Sergio
 Sent to CCL by: Tom Albright [talbright1234]![gmail.com]
 Hi Sergio - the best way (I think) to look at metal carbenes is like you stated
 before, a filled sp2 type orbital on CH2 which interacts with some hybridized,
 empty orbital on the metal. This will form the sigma bond which might be
 slightly polarized more or less on the carbene side (that will depend upon the
 metal, how electronegative the ligands are around the metal and, of course for
 real carbenes what substituents are in place of the H's) But the important
 factor in terms of reactivity - because the Schrock versus Fischer
 classification is used to describe the reactivity at the carbene center - comes
 from the pi portion of the M=C bond. In simple terms one can think that the d
 orbital at the  metal has two electrons and lies at a lower energy than the
 carbene p AO. Then the carbene carbon will be electrophilic, i.e. more of the
 electron density lies on the metal side for the M=C pi MO and the coefficient in
 the M=C pi* MO is larger on the carbon. This is the essence of a!
   Fisher carbene. Conversely if the d orbital is at higher energy than carbon p
 AO, then the bonding situation can be reversed so that the carbon is now
 nucleophilic - a Schrock carbene. The differences here often lie with the
 auxiliary ligands at the metal, e.g. Fischer carbenes often have electron
 withdrawing CO ligands; Schrock carbenes have alkyl groups and/or phosphines at
 the metal. Fisher carbenes typically have the N(R)CH2CH2N(R) group connected to
 the carbene carbon so the pAO - centered MO lies at high energy whereas
 Schrock's have simple alkyl groups. I hope this helps...
 With Best Regards
 Tom
 On Sep 25, 2014, at 2:13 AM, Sergio Manzetti sergio.manzetti[*]outlook.com
 wrote:
 >
 > 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 types?
 >
 > I haven't read you book yet, I will get there soon.
 >
 > Best wishes
 >
 > Sergio
 >
 >
 >
 > 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 numbers.
 >
 > 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 mechanism.
 >
 >    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.
 >
 >    Sergio
 >
 >        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]
 >
 >        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,
 >
 >        Fedor
 >
 >
 >        On 9/18/2014 10:44 AM, Sergio Manzetti sergio.manzetti a outlook.com
 wrote:
 >
 >            Sent to CCL by: "Sergio Manzetti"
 [sergio.manzetti!=!outlook.com]
 >
 >            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.
 >
 >
 >            Thanks>
 >
 >
 >
 >
 >        --
 >
 >        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**scm.com
 >
 >        http://www.scm.comthe strange characters on the top line to
 the ,, sign. You can alsoE-mail to subscribers: CHEMISTRY,,ccl.net or use:E-mail
 to administrators: CHEMISTRY-REQUEST,,ccl.net or use<brIf your mail bounces
 > from CCL with 5.7.1 error, check:With Best Regards
 > Tom Albright
 >
 >
 >
 >
 > Too many newsletters? You can unsubscribe.http://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp-:-//www.ccl.net/chemistry/sub_unsub.shtmlhttp-:-//www.ccl.net/spammers.txtToo
 many newsletters? You can unsubscribe.
 HP EliteBook Folio 1040
 HP EliteBook Folio 1040 with Windows 8.1 Pro.
 #archiveModalDialog