Organometallics Flashcards
Draw and name the bonding between a metal and a CO ligand. Why does this allow many different complexes to exist?
Synergic bonding. It can stabilise both high and low oxidation states.
How do you count the electrons of a metal and complex?
Find the group of the metal, this gives the number of d electrons. Work out the oxidation state from the charged ligands. Adjust the d electrons to account for electrons lost or gained from the oxidation state, this gives the final electron count. Add the electrons due to the ligands, some ligands have different possible donation numbers.
Describe the process of oxidative addition. Give the names of the three mechanisms it can occur by.
A metal reacts with an incoming ligand, such as H2 or any H-R or X-R molecule, and is oxidised twice to with two negative ligands becoming bonded to the metal.
Concerted oxidative addition, heterolytic splitting and homolytic splitting (or atom transfer)
Describe the process of concerted oxidative addition including the metal requirements (and what can happen if they’re not met), the transition state, the rate, stereochemistry and the activation parameters. Use H2 as an example.
The H2 adds to the metal in one step and the metal is oxidised twice. A 16 electron complex which can accept two extra ligands and can lose two electrons is required. A reaction (such as a reflux) may be required for a suitable reactant complex. If the metal is not electron rich enough to populate the σ* orbital and break the σ bond, the H2 can act as a single 2 electron donor ligand.
In the transisition state the H-H σ bond lengthens as the M-H bond forms and the other ligands bend away. The rate = k[M][H2] as the reaction is concerted. The activation parameters are an endothermic barrier due to breaking the H-H bond and a negative entropic reaction (approx -100). Stereochemistry is retained if the new ligands are chiral and they are always cis to each other.
Describe the mechanism of heterolytic splitting for oxidative addition. Describe the activation parameters and 4 differences from the concerted process
The molecule, such as MeI, splits into a positive and negative component with the positive fragment interacting with the metal. The metal is oxidised twice and the fragment becomes negatively charged. The reaction can stop here with the negative charged species as a counter-ion, or the negatively charged fragment can then add to the complex.
The activation parameters are endothermic and about -200 J/Kmol change in entropy due to the solvent effects.
Differences from the concerted mechanism include: faster rate in polar solvent, no change in oxidation state on second addition, inversion of stereochemistry and the new ligands can be cis or trans.
Describe the radical mechanism, homolytic splitting, of oxidative addition and how it compares to the other mechanisms.
The A-B incoming molecule is homolytically split and oxidises the metal once each. The reaction is rare and hard to prove has occured. Due to the radical intermediate, stereochemistry is lost and the two groups can be cis or trans.
Describe reductive elimination and how the mechanism can be favoured.
Reductive elimination which is favoured by warming the complex as this favours the increase in entropy.
One centre reductive elimination is the reverse of oxidative addition where the A-B bond is formed. The groups must be cis. The reaction is more likely when the complex is in an unstable oxidation state.
Describe migration reactions and explain how the electron count changes. What ligands can this occur to?
An 18 electron complex, has a reaction between its ligands to form a new ligand and a 16 electron complex. Another incoming ligand then reacts and forms an 18 electron complex again.
Methyls can migrate onto a CO ligand to form a carbonyl ligand. Hydride can migrate onto an alkene (forming a ligand with the π bond) to form an alkyl ligand with the migrated hydrogen on the end. If another alkene adds to the complex an alkyl migration can occur to form an even longer alkyl group. This has implications on polymer catalysis.
Draw a reaction scheme for how TiCl3 can heterogeneously catalyst alkyl chain growth and describe the properties of the reaction and products.
The reaction must be dry as water will attack the Ti centre. Termination occurs through beta-hydrogen transfer of water or H2 oxidative addition.
The direction of addition is the same every time and the chain is isotactic (Me on the same side).
Describe the bonding of alkyl ligands and how the R-M bonds can be synthesised.
They are a strong bond between the sp3 carbon and an empty d orbital. They are R- groups and are 2 electron donors. Bonds can break by homolytic splitting but can be strong.
Synthesis can be via: oxidative addition with R-I (weakly bonded) via the heterolytic pathway, by nucleophilic attack and substitution using Me-Li+ (favours forming LiCl salt), electrophilic attack with MeI and a negative complex to form a salt with I- and the complex counter-ion.
Name and describe the three ways that alkyl ligands decompose.
- beta-hydrogen elimination - a hydrogen becomes a hydride ligand and forms an alkene which is bonded to the metal from its π orbitals. This then dissociates due to the weak bonding. This can be stopped by saturating the metal centre, removing any beta hydrogens and making the alkene formation unfavourable.
- alpha hydrogen elimination - an alpha hydrogen elimination to form a hydride and a carbene ligand. This has poorer orbital overlap than beta elimination but often occurs when another alkyl ligand is present which can be liberated with the hydride as an alkane gas.
- Homolytic cleavage - radicals form on the metal and carbon. Very rare mechanism
Draw a diagram to show and describe agostic complexes highlighting their electron counts. Draw their bonding and describe how they can be characterised. When are they common?
Agostic complexes occur where beta (and sometimes alpha) hydrogen transfer halts halfway through and a ligand forms from the C-H bond which is transferring. Two electrons are added to the electron count for the metal. The bonding is strong as it is synergic and can recieve back bonding.
The IR and NMR ranges for the C-H bond drastically change in the agostic complex.
Agostic complexes are common for electron poor metal centres and the C-H bond breaks as the metal becomes more electron rich.
What can you do to make a complex that is prone to oxidative addition?
Some ligands such as PPh3 are weakly bound so they can bind reversibly to a metal which can make room for incoming ligands at times.
Oxidative addition is good to 16 electron complexes including square planar complexes which are often stable
How can an oxidative addition, then a reductive elimination reaction occur in a molecule such as IrMe(PPh3)3 with no other reagents?
The 16 electron complex undergoes oxidative addition with a C-H bond on one of the phenyl groups making an 18 electron complex with a hydride and an phenyl bond. Reductive elimination can then occur between the methyl group and the hydride to make a 16 electron complex.
What roles do bulky ligands such as Cp take in catalysis? How can they be manipulated to make a better catalytst?
They protect the metal with steric bulk. The large groups can be linked with an alkyl chain to add steric strain which makes the complex more reactive and highly specific for catalysis such as creating highly isotactic polymers.
What is the shape of carbene orbitals and how are they stabilised?
The carbene is an sp2 orbital with a pz orbital. The singlet states are where the 2 electrons occupying the orbitals are either both in the sp2, or one in each with opposite spins.
Carbenes are stabilised by electron donation into the pz orbital from nitrogen or other π donors.
Draw the structures and outline the synthesis of the Fischer and Schrock carbenes.
Fischer: W(CO)6 attacked by Ph- at a CO to form a carbonyl type ligand. Source of Me+ introduced which attaches to the O.
Schrock: Cp2TaMe3 has Me removed by CPh3, then sodium methoxide removes a hydrgen from one methyl to make the carbene.
Draw the orbital structure of carbenes, comparing Fischer and Schrock, and describe the charge and how many electrons are donated of each.
Fischer: must have a π donating R group as seen in 3, not much electron density transferred from the metal to the carbon. Therefore it is a netural 2 electron donor.
Schrock: pz only recieves electron density from the metal, formally 2 electrons. Therefore it is a 2- charge, 4 electron donor.
Draw an example of an alkene methathesis reaction and the Schrock and Grubbs catalysts. Compare the properties of the two catalysts.
Schrock: Very active but very sensitive to air and water, can’t be used with esters or alcohols. This is due to its high oxidation state.
Grubbs: Less active but more tolerant to impurities and functional groups.
Give the Chauvin mechanism for catalysed metathesis.
Draw the mechanism for ring-closing metathesis using RuCl2(PCy3)2CHR and a general diene.
Draw the orbitals of alkyne donation to metals and describe how you count the electrons that it contributes to a metal.
The alkyne can donate 2 or 4 electrons to the metal. This can even be shared between metals for 2 each. To work out how many electrons are donated to the metal, work out the electron count before the alkene is added, then add them up to 18.
Describe the NMR spectrum of the eta-3 allyl ligand and draw the protons interchange mechanism.
At low temp. a triplet of triplets is seen for the middle proton with two doublets for the outer protons. One proton from each of the outer carbons forms the signal at each doublet. As the temperature increases, the middle proton turns into a multiplet and the outer proton peaks average out.
Draw a cycle and give reagents to show how allyl ligands can form new C-C bonds with Pd(Ph3)2. How can the reaction be made to be purely one enantiomer?
By using a chiral phosphine, the enantio pure product can be formed. This is used extensively in industry.
