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Structure, Properties and Bonding of Organometallic Compounds Dr. Christoph Jan.2012 Slide 2 What is Organometallic Chemistry ? Slide 3 Slide 4 Main Group Compounds http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx4.htm 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: Slide 5 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 ! Slide 6 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 Slide 7 Creation of carbon-nucleophils Slide 8 More on: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx4.htm Slide 9 Transition Metals Low EN High Oxidation States hard metal centers Higher EN Low Oxidation States soft metal centers 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 ! Slide 10 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 ! Slide 11 Slide 12 Hapticity (eta) For example with Cyclopentadiene: Slide 13 Bridging Ligands () Slide 14 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 ! Slide 15 (2) Five coordinate (ML5) Trigonal bipyramidal Square pyramidal (rare !) Slide 16 (3) Four coordinate (ML4) Tetragonal (mostly 1 st row Transition Metal) Square planar (2 nd and 3 rd row Transition Metal) Slide 17 Metal-Ligand Bonds (1) -donor Ligands Slide 18 That means also that a ML6 molecule can exist with 12 valence electrons up to 22. The best situation is to have 18 electrons ! Slide 19 Group Orbitals from ligands: For a 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 AOs spherical around the center of an octahedron. Slide 20 Symmetry of orbitals depending 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 AOs) 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 ! Slide 21 Slide 22 Strong and weak donors weak Slide 23 Bonding and Group Orbitals Part 2 of Organometallic Compounds Slide 24 CH 4 molecule Fill the electrons and mark the HOMO and LUMO. Draw the MOs of the HOMO and LUMO as well. We combine the 4 Hydrogen AOs 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) Slide 25 http://firstyear.chem.usyd.edu.au/calculators/mo_diagrams.shtml Slide 26 http://www.webqc.org/symmetry.php The symmetry of the 3 combinations of H-AOs in CH4 => have the same energy, the representation t2 and match with the 3 p-orbitals of Carbon ! Slide 27 Complete the MO Diagrams on the website for: 1.CN(-) 2.CO 3.NO 4.NO(+) 5.CH4 6.BH3 7.SF6 8.Fe(OH2)6 (3+) 9.TiCl4 10.CuCl4(2-) http://firstyear.chem.usyd.edu.au/calculators/mo_diagrams.shtml Slide 28 Paramagnetic O 2 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 MOs. Slide 29 This MO is lower in energy than the - orbitals because of bigger overlap. Different from N2, this p-MO is not pushed up in energy by the lower s-MO which is antibonding ! Slide 30 Slide 31 [Co(NH 3 ) 6 ] 3+ as an example Six LGO (Ligand group orbitals) have symmetries that match the s, p and dz 2 and dx 2 -y 2 orbitals. Slide 32 Where is the energy difference (between which orbitals) that corresponds to D o (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. Slide 33 (2) -Donor Ligands => Ligands like Cl - are both -donor AND -donors Slide 34 Filled ligand orbitals change non-bonding metal d-orbitals into antibonding => Raising of the t2g level and reducing the crystal field splitting energy Slide 35 Multiple bonds as - donors A new interaction comes up ! One of the former non-bonding t2g d- orbitals can now interact with an empty * MO of ethylene ! Slide 36 The Donation bond causes no rotation barrier, but the back-bonding does ! Slide 37 Dewar-Chatt-Duncanson Model Slide 38 M-ethylene bonds Slide 39 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 ! Slide 40 Nucleophilic attack on C -H-Elimination Example: Nu = H2O Wacker Process to create Acetaldehyde Slide 41 Electrophilic attack on Metal Example: Shell Higher Olefin Process SHOP = Oligomerisation of Ethylene Slide 42 Slide 43 Cp(-) as ligand Slide 44 This type of molecule is more interesting as catalyst because it can easily lose/replace a Cl- ligand Slide 45 Transition metal Cp Complex Families Slide 46 (3) -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 ! Slide 47 Which additional interactions do we get If the ligands have empty * orbitals ? For example with CO: ** Electrons from a metal d- orbital (t2 set) can go into the antibonding MO of a CO ligand ! We call this -backbonding ! Slide 48 Each CO ligand has two empty * 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 -donor as well ! Slide 49 -Backbonding Slide 50 Ligands with -Backbonding Slide 51 3 combinations of * CO ligands can overlap with the 3 d-orbitals of t 2g (d xy, d xz,d yz ) Slide 52 CO Complexes as starting materials for synthesis (a) Thermal CO substitution (b) Photochemical CO substitution Slide 53 Complete the MO Diagrams on this website for different molecules as group work (see list on the office door) Draw on paper or print it out -> to office SC 2201 please until 24.1.2555, thank you ! http://firstyear.chem.usyd.edu.au/calculators/mo_diagrams.shtml Homework get some extra points for the final exam ! Slide 54 Thank you for your patience ! This subject is really hard to understand in a short time. Therefore I suggest we will have a make-up lesson in February. If you are interested in catalysis, please contact me or another teacher for a possible project in the lab thank you ! Thank you for your patience ! This subject is really hard to understand in a short time. Therefore I suggest we will have a make-up lesson in February. If you are interested in catalysis, please contact me or another teacher for a possible project in the lab thank you !