Metal-Carbon π-Bonds Part 3 & Catalysis

• The bonding in arene-metal complexes is qualitatively very similar to metallocenes, but since the arene is neutral, all the bonding electrons comes from the arene • Because there is no charge to the arene, there is less electrostatic contribution to the bonding and hence metal-arene complexes are LESS stable than metallocenes/Cp complexes

• σ-bonding (1 nodal plane) • π-bonding (2 nodal planes) • δ-bonding (3-nodal planes)

• The 6MR aligns better with the metal d-orbitals, resulting in much better orbital overlap between d-orbitals on metal and p-orbitals on carbon • resulting in stronger back donation • Resulting in the arene being electron rich and the metal being δ+

Reduce a metal halide salt in the presence of benzene to for the bis-(benzene) complex

Co-condensation of metal and ligand vapours allows access to bis-(arene)metal complexes inaccessible by other mean We can vaporise our metal and introduce an aromatic ring in the gas phase

We can replace two free electron donor with an aromatic ring

True We can exchange arene rings if the displacing arene is more electron donating (e.g. with Me substituents)

an arene half-sandwhich complex and aliminium tetrachloride (from the chloride which is displace)

• The Cr(CO)₃ fragment is strongly electron withdrawing • Which renders the arene electron poor (electrophilic) • Hence nucleopilic substitutions reactions are fast (and electrophilic substitutions are slow) • Also results in protons on the aromatic ring becoming more acidic, so deprotonation becomes easier

• Cyloheptatriene is η6, hence has 6 pi-electrons corresponding it to an aromatic system • Hence treating it with the following carbocation will remove a hydrogen • The formed cycloheptatrienyl formed with cordinate η7 to the metal centre

• treating it with butyl lithium will deprotonate it, which pushes the electrons into the ring • Forming an C₇H₇⁻ complex which has 8 pi-electrons and is not an aromatic system • Can be stabilised by binding η3 to a metal centre however

• It has 8 pi-electrons • doesn’t obey the 4n+2 rule

• By reduction using 2e- forming C₈H₈²⁻ • Meaning it has 10 pi-electrons and is aromatic • When aromatic is can coordinate through all 8 of the carbons

Large metals Due to its own large size (e.g. this uranium-cyclooctatetraenyl complex due to uranium being a large metal) It is also not an 18e species because it is an f-block element

• One will coodinate η8 • and one will coodinate η4 (neutral/non-aromatic species)

• Sometime organic ligands are continuously reorganising their position at a metal centre • At higher temperature NMR they covert so fast they cannot be distinguished but at lower temperature you can maybe distinguish multiple positions

True

• Interchange of two or more configuations which are chemically inequivalent (e.g. η5 → η1 in Cp complexes or η3 → η1 in metally allyl complexes) • Interchange of two or more configurations which are chemically eqivalent e.g. 1,2- or 1,3-shifts of η1-Cp ligands

With increasing temperature, there can be a switch from C1 to C2 (1,2-shift) or C1 to C3 (1,3-shift) being found to the metal centre (results in the NMR signal start to broaden out)

• Ligand dissociation • Triphenylphosphate is lost

• Oxidative addition of H₂ • Going from 14e- to 16e (more stable)

• Coordination of an alkene • Going from a 16e to 18e complex (more stable)

• 1,2-migratory insertion of the alkene • Removal of the C=C bond

• Reductive elimination of the alkane • Final alkane product has been formed and the 14e Rh complex has been reformed