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Structure, Properties and Bonding of Organometallic Compounds

Structure, Properties and Bonding of Organometallic Compounds. Dr. Christoph Jan.2012. What is “Organometallic Chemistry” ?. Main Group Compounds. Alkali and earth alkali metals have a high attraction to halogens .

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Structure, Properties and Bonding of Organometallic Compounds

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  1. Structure, Properties and Bonding of Organometallic Compounds Dr. Christoph Jan.2012

  2. What is “Organometallic Chemistry” ?

  3. Main Group Compounds Alkali and earth alkali metals have a high attraction to halogens. They can even detach a halogen atom from an organic C-X bond and “insert” into this bond: Reaction with dihalides can form double bond: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx4.htm

  4. The metal can be: Li , Na, K , Mg, Ca, and also Sn and Zn(metals must be clean and have big surface) The reactions take place in Ether or THF, free of water !

  5. Grignard Chemistry Grignard Reagents – creating carbanions for nucleophilic attack reactions Mg Powder in Ether or THF reacts with R-X compounds (aliphatic and aromatic) by “inserting” in the C-X bond. http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/substitution/alkylhalogenide/metallorg_verbindungen.vlu/ Page/vsc/en/ch/12/oc/substitution/alkylhalogenide/organometall2/organometall2.vscml.html

  6. Creation of carbon-nucleophils

  7. More on: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx4.htm

  8. Transition Metals Note that we count the two s-electrons of the atom together with the d-electrons ! For example Ti atom has 4s2, 3d2 BUT we count it as 4 d-electrons in a molecule with Ti ! • Low EN • High Oxidation States • “hard” metal centers • Higher EN • Low Oxidation States • “soft” metal centers

  9. First organometal compounds Note that the ethylene molecule is NOT flat anymore but has some sp3 hybridization on the carbons ! That means also that the C=C bond is not a complete double bond anymore !

  10. “Hapticity” η (eta) For example with Cyclopentadiene:

  11. Bridging Ligands (μ)

  12. Common Coordinations (1) Six coordinate - octahedral (ML6) but also: Cp(-) bonds with 3 electron pairs to the metal (similar to benzene) => Cp(-) counts as 3 ligands !

  13. (2) Five coordinate (ML5) Trigonal bipyramidal Square pyramidal (rare !)

  14. (3) Four coordinate (ML4) Tetragonal (mostly 1st row Transition Metal) Square planar (2nd and 3rd row Transition Metal)

  15. Metal-Ligand Bonds(1) σ-donor Ligands

  16. That means also that a ML6 molecule can exist with 12 valence electrons up to 22. The best situation is to have 18 electrons !

  17. Group Orbitals from ligands: For a σ donor ligand, we consider just the electron pair that donates electrons . We combine 6 ligand orbitals (whether in s- or p-AO does NOT matter !) to 6 SALCs(Symmetry Adapted Linear Combination) SALC = combination of the 6 ligand AO’s spherical around the center of an octahedron.

  18. Symmetry of orbitalsdepending on the point group In an octahedral molecule Oh the s-orbital has A1 symmetry, the p-orbitals form a set of T1 orbitals, the d-orbitals have 2 different sets of symmetry: Eg and T2g (2 plus 3 AO’s) We can combine the 6 ligand orbitals around the center in the way that we get a match with A1, with T1 and with Eg – the T2g orbitals of the metal do not find a partner here !

  19. Strong and weak σ donors weak

  20. Bonding and Group Orbitals Part 2 of Organometallic Compounds

  21. CH4 molecule Fill the electrons and mark the HOMO and LUMO. Draw the MO’s of the HOMO and LUMO as well. We combine the 4 Hydrogen AO’s together with the symmetry of the molecule (Td). One combination has no node (low energy), three combinations have each one node plane (set of 3)

  22. http://firstyear.chem.usyd.edu.au/calculators/mo_diagrams.shtmlhttp://firstyear.chem.usyd.edu.au/calculators/mo_diagrams.shtml

  23. http://www.webqc.org/symmetry.php The symmetry of the 3 combinations of H-AO’s in CH4=> have the same energy, the representation t2 and match with the 3 p-orbitals of Carbon !

  24. http://firstyear.chem.usyd.edu.au/calculators/mo_diagrams.shtmlhttp://firstyear.chem.usyd.edu.au/calculators/mo_diagrams.shtml Complete the MO Diagrams on the website for: CN(-) CO NO NO(+) CH4 BH3 SF6 Fe(OH2)6 (3+) TiCl4 CuCl4(2-)

  25. Paramagnetic O2 Draw a MO diagram for oxygen and explain why it is a di-radical and therefore paramagnetic ! Mark the HOMO and LUMO and draw the MO’s.

  26. This MO is lower in energy than the π-orbitals because of bigger overlap. Different from N2, this σp-MO is not pushed up in energy by the lower σs-MO which is antibonding !

  27. [Co(NH3)6]3+ as an example Six LGO (Ligand group orbitals) have symmetries that match the s, p and dz2 and dx2-y2 orbitals.

  28. ? Where is the energy difference (between which orbitals) that corresponds to Do (10Dq)? Which orbitals are anti- and which are non-bonding? Fill the electrons into the labels and count the total number. Mark the LUMO and HOMO.

  29. (2) π-Donor Ligands => Ligands like Cl- are both σ-donor AND π-donors

  30. Filled π ligand orbitals changenon-bonding metal d-orbitals into antibonding => Raising of the t2g level and reducing the crystal field splitting energy

  31. Multiple bonds as σ- donors A new interaction comes up ! One of the former non-bonding t2g d-orbitals can now interact with an empty π* MO of ethylene !

  32. The “Donation” bond causes no rotation barrier, but the back-bonding does !

  33. Dewar-Chatt-Duncanson Model

  34. M-ethylene bonds

  35. Bonding Situation => Reactivity ! Depending on the metal and the other ligands on the metal center, the “real” situation for an olefin complex is in between these extremes !

  36. Nucleophilic attack on C β-H-Elimination Example: Nu = H2O Wacker Process to create Acetaldehyde

  37. Electrophilic attack on Metal Example: “Shell Higher Olefin Process SHOP” = Oligomerisation of Ethylene

  38. Cp(-) as ligand

  39. This type of molecule is more interesting as catalyst because it can easily lose/replace a Cl-ligand

  40. Transition metal Cp Complex Families

  41. (3) π-acceptor Ligands Because of the symmetry of the p-orbital, it is also possible that a metal can push electrons into an empty p-orbital of a ligand, which is normally anti-bonding for the ligand molecule! => the bond in the ligand becomes weaker then !

  42. Which additional interactions do we get If the ligands have empty π* orbitals ?For example with CO: π* Electrons from a metal d-orbital (t2 set) can go into the antibonding MO of a CO ligand ! We call this π-backbonding!

  43. Each CO ligand has two empty π* orbitals. They can accept electrons from a metal d-orbital (dxy, dxz, dyz) ! Because of the HOMO the CO ligand has high electron density on the carbon => can act as σ-donor as well !

  44. “π-Backbonding”

  45. Ligands with π-Backbonding

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