Metal-Carbon σ-bonds

1) Salt Elimination 2) Oxidative addition 3) Alkene Insertion 4) Ortho-metallation

• Low-valance complexes (e.g. Ir(I), Ni(0), Pd(0), and Pt(0) ), which are stabilised by phosphines undergo oxidative additions with alkyl, benzyl and aryl halides • Classically is a d⁸ square planar complex that does oxidative addition to give an octahdedral complex

Increasing covalent character of the M-C bond (less covalent = less reactive) E.g. MeLi is more polarised than AlMe₂Cl and hence will methylate 4 times compared to once for AlMe₂Cl

• Hydride or an alcohol chain inserting onto a coordinated alkene to give you an alcohol chain on the metal centre • (Crucial step in the polymersation of alkenes)

• Involves the cleavage of the ortho-C-H bonds of an aryl (usually phenyl) • (it is a common deactivating pathway of noble metal phosphine catalysts)

• To form new bonds with electrophiles e.g. halogen • hydrogenolysis (forming C-H bonds)

• React with CO to add carbonyl groups between the M-C bond • React with alkenes, to extend the carbon chain, between the M-C • React with alkynes to add a C=C between the M-C

False Metal hydrides are similar to small alkyls which we cannot elimated

• Beta-hydride elimination • Hydrogenolysis • Oxidative addition of H₂

• The hydrogen has a filled sigma bonding orbital and can donate electron density to the metal centre (σ-donation) • Also if metal has filled metal d-orbitals and hydrogen has empty σ’ orbital, there is π-back donation • However the π-back donation will weaken the H-H bond untill is breaks - also known as oxidative addition

• Schrock Carbenes have early transition metals which are electron poor and high in oxidation state • their carbons are nucleophilic • their ligands are π-donors • They have two unpaired electrons in different orbitals (called a triplet binding)

• The carbon is acting as a two electron donor: ne electron through the sigma systen and one electron through the pi system • In total 4 electrons are involved in the bonding between the metal and carbon

• Fischer Carbenes tend to be formed with late transition metals that are electron rich and hence a low oxidation state • Carbon is electrophilic (neutral ligands resulting in a δ⁺ carbon centre as metal is more δ⁻) • Ligands are π-acceptors • Ligands will donate their LP from p-orbitals into carbons empty p-orbitals, meaning carbon electrons for the C-M bond are actually paired, called a singlet binding

• sp² hybrided carbon • The LP on the carbon will donte to the metal • The metal can then back-donate a pair of electron from a d-orbital into an empty p-orbital on carbon

• α-hydrogen abstraction • a hydride from one of the ligands, transfers and attacks the other ligands • This breaks one Ta-C bond and allows a new Ta=C bond to form • Eliminating tBuCH₃

It has 5 alkyl groups on it which do not have a beta-hydrogen Instead all have a tert-butyl group in that poisition

• Usually from metal hexa-carbonyl species reacting with a nucleophile • Where the nucleophile attacks one of the C=O ligands • Then we alkylate (e.g. triethyloxonium)

• It is good at transferring CR₂ groups onto other things • E.g. reacting this Tantalum Schrock with a ketone we can form this alkene species

• They are highly reactive • Meaning many of these Schrock carbenes species (e.g. a titanium carbene species) cannot be isolated in its pure form

• By using Tebbe’s reagents • It is a methylene-transfer reagent (an alternative to a Wittig reagent), and it can be regarded as a protected form of [Cp₂Ti=CH₂] which is unstable when unprotected

Using an Amine

• A [2+2] cycloaddition reaction of a carbene with an alkenes give a metallacyclobutane (reversible)

• Relatively stable carbene with a lone pair • Stabilised through the nitrogen atom(s)

Nitrogen lone pairs can donate into the p-orbital on carbon, to stabilise it Making it favourable for the 2E- on carbon to pair up in the sp² lobe (donate to a metal) singlet diamagnetic ground state (Note the R groups can also help stabilise due to being bulky and electron donating)