Structure, Properties and Bonding of Organometallic Compounds Dr. Christoph Jan.2012
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
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 !
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
More on: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx4.htm
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
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 !
“Hapticity” η (eta) For example with Cyclopentadiene:
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 !
(2) Five coordinate (ML5) Trigonal bipyramidal Square pyramidal (rare !)
(3) Four coordinate (ML4) Tetragonal (mostly 1st row Transition Metal) Square planar (2nd and 3rd row Transition Metal)
That means also that a ML6 molecule can exist with 12 valence electrons up to 22. The best situation is to have 18 electrons !
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.
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 !
Bonding and Group Orbitals Part 2 of Organometallic Compounds
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)
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 !
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-)
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.
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 !
[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.
? 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.
(2) π-Donor Ligands => Ligands like Cl- are both σ-donor AND π-donors
Filled π ligand orbitals changenon-bonding metal d-orbitals into antibonding => Raising of the t2g level and reducing the crystal field splitting energy
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 !
The “Donation” bond causes no rotation barrier, but the back-bonding does !
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 !
Nucleophilic attack on C β-H-Elimination Example: Nu = H2O Wacker Process to create Acetaldehyde
Electrophilic attack on Metal Example: “Shell Higher Olefin Process SHOP” = Oligomerisation of Ethylene
This type of molecule is more interesting as catalyst because it can easily lose/replace a Cl-ligand
(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 !
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!
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 !