CCL: Metal-Carbene sigma type bond

 Sent to CCL by: "Sergio  Manzetti" [sergio.manzetti]^[]
 Dear Alan and Fedor, I ended up with two potential schemes:
 The sigma-bond scheme according to the Fischer model:
 The Tantalum atom donates two 5dxy electrons which pair with the carbons pi*
 antibonding orbital. And the carbon donates two sp2 electrons to the 5dy^2 empty
 orbital in Tantalum.  The 5dy^2 is expected to have the highest energy of all
 the interacting Tantalum orbitals and the pi* of carbon (perpendicular to the
 double bond pairing plane) naturally is higher than the sp2.
 Would this sound reasonably as a non-polarized bond?
 Sketching below the Fichers alternative (up electron -u down electron -d):
              /      \
             /   ____ \
 5dy^2 _____/ /      ' \
            \/         '\
 5dxy  _u_d_/\           \' _____  pi* (3px/y)
            \ \           \'
             \ \         ' \_u_d_  Csp2
              \ \_u_d_ -'--/
               \       '
 And the Schrock alternative:
              /      \
             /   ____ \
 5dx-z __u__/ /      ' \
            \/         '\
 5dx-y __u__/\           \' __d__  Cp-z
            \ \           \'
             \ \         ' \__d__  C-sp2
              \ \_u_d_ -'--/
               \       '
 The question is, if these are correct: would there be a way to know which of the
 two is likely for heacvy and which is more likely for light transition metals?
 The Ta=CH2 system (Schrock carbene) usually is viewed as containing two polar
 bonds. Both the sigma and pi bonds can be viewed as polarized Ta+  C-.
 In your description, you have a three electron sigma bond and an one electron pi
 bond (composed of "an empty 2py of the carbon interacting with ONE of the
 5d electrons").
 Instead, do this:
 sigma: pair 1 C 2px with Ta 6s. Polarize this pair towards C
 pi: pair 1 C 2py with 1 Ta 5d. Polarize this pair towards C too
 On Thu, Sep 18, 2014 at 1:44 AM, Sergio Manzetti sergio.manzetti a
 <owner-chemistry!^!> wrote:
     Sent to CCL by: "Sergio  Manzetti" [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.
     E-mail to subscribers: CHEMISTRY!^! or use:E-mail to administrators:
 CHEMISTRY-REQUEST!^! or use<br--
 Alan Shusterman
 Chemistry Department
 Reed College
 3203 SE Woodstock Blvd
 Portland, OR 97202-8199
 "Nature doesn't make long speeches." Lao Tzu 23
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