From owner-chemistry@ccl.net Sat Sep 20 10:26:00 2014 From: "Sergio Manzetti sergio.manzetti[#]outlook.com" To: CCL Subject: CCL: Metal-Carbene sigma type bond Message-Id: <-50522-140920045001-15048-jShlJeYtnjKXGtLCg6zBpQ^^^server.ccl.net> X-Original-From: "Sergio Manzetti" Date: Sat, 20 Sep 2014 04:49:58 -0400 Sent to CCL by: "Sergio Manzetti" [sergio.manzetti]^[outlook.com] 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_ -'--/ \ ' \_u_d_' And the Schrock alternative: ____ / \ / ____ \ 5dx-z __u__/ / ' \ \/ '\ 5dx-y __u__/\ \' __d__ Cp-z \ \ \' \ \ ' \__d__ C-sp2 \ \_u_d_ -'--/ \ ' \_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? Cheers Sergio ____________ 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 Alan On Thu, Sep 18, 2014 at 1:44 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