L4: Metal alkyl complexes Flashcards
Trends in M-C bond dissociation energies
- Down a d-block group, bond strengths generally increase (lower dissociation energy)
- This is due to relatively small size of the metal’s valence d-orbitals
- (n-1)d, ns and np become more similar in size as we go down the group and this results in better overlap with ligand orbitals -> stronger bonding
- The opposite trend occurs down a main group
Decomposition routes in main group (x1) vs transition metal (x2), consequence for synthesising robust, stable complexes
- In main group, only s and p v.o present which are filled (i.e. octet rule obeyed) so the only likely route to initiate a thermal decomposition is a relatively high-energy process
- In t.metals, however, there are empty v.o’s and free coordination sites allowing much lower-energy routes to set off thermal decomposition
e.g. beta-hydrogen elimination - This must be prevented to synthesise robust, kinetically stable t.metal alkyls
- Also, bond homolysis can occur (radical)
Key requirements for beta-hydrogen elimination (x4)
- A hydrogen atom on the beta-carbon of the alkyl group
- A vacant coordination site cis to the alkyl group
- A vacant v.o on the metal atom (therefore must be 16e- species)
- Co-planarity of the M-C-C-H unit (‘syn-periplanar’) - excludes species like adamantyl with rigid rings keeping M-C-C-H dihedral angle distorted
Issue with beta-hydrogen elimination for phenyl/vinyl complexes
- Tend to be more robust than alkyl complexes but their beta-H’s don’t eliminate easily
- M-C and C-H bonds are stronger
- 120 degree bond angle at C keeps beta-H further away from metal; less favourable
Arrested beta-H elimination
- B-H held halfway, bridges between metal and B-carbon in a 3-centre 2-electron interaction
- e.g. Ti(IV) where there are no d electrons so no back-bonding from M, which would help break the C-H bond and also stabilise alkene coordination
Alpha-elimination
- When B-H elimination can’t occur
- Leads to formation of alklyidenes (carbenes) w/ M=C double bonds
- May also be involved in the decomposition of methyl complexes
Reductive elimination requirements (x3) + process
- cis arrangement of groups to be eliminated
- Occurs more readily if metal OS 2 units below is stable
- usually more rapid if R-H rather than R-R can be eliminated
Two alkyl groups or an alkyl and a hydride ligand are eliminated in a concerted reaction
The 2 ligands lost are both formally anionic so the OS of the M is reduced by 2 units
Synthesis of metal alkyl complexes
- Alkylation of metal halides
Reagents w/ a nucleophilic alkyl group have been used (e.g. Grignard) - Metal carbonyl anion plus an alkyl halide (Nu- attack)
- Nucleophilic addition to alkene complexes
- Insertion of an alkene into an M-H bond
- Oxidative addition of an alkyl halide to a 16-electron complex (adding X-Y)
- Reductive elimination
Requirements for oxidative addition (x3), typical OA reactive complex
- Metal centre in low oxidation state
- Coordinatively unsaturated
- Electron rich (‘late’ metal, phosphine ligands preferable to CO)
- Typically LnM complex is square planar with 16-electron count
Mechanisms for oxidative addition (x3)
- Concerted
Likely to operate if X-Y is non-polar. Always a cis addition. e.g. of H2 - Stepwise
M acts as a lewis base (nucleophile) in 1st step. There will be an inversion of configuration at the alpha-C if R-X is chiral. R and X end up cis or trans to each other - Radical
Rare
Thermodynamics in oxidative addition (2 contributing factors)
Product increasingly favoured as…
- L becomes more electron-donating
- X becomes less electron-withdrawing (Cl<Br<I)
Reagents for alkylation of metal halides
- RLi
- RMgX
- R2Mg
- AlR3
- ZnR2
- Only the (halide) groups are alkylated
Reactivity of t.metal alkyls towards electrophilic reagents
- ‘Grignard-like’
- Results in M-C bond cleavage
- Early metals are more electropositive so react vigorously
- Late metals react more smoothly
Carbonyl insertion (About, including product, mechanism, configuration)
- Converts alkyl complex into an acyl complex
- Reversible (‘extrusion’)
- Other incoming ligands should also promote CO insertion
- In the proven mechanism, the alkyl group migrates in the first step (studying reverse reaction) -> intramolecular nucleophilic attack by R (delta-) on the CO ligand
- If R is a chiral group, there is a retention of configuration
Products of carbonyl insertion with coordinating vs noncoordinating solvent
- Investigating retention vs inversion of configuration
- Get mostly inversion for non-coordinating solvent (EtNO2) -> expected for R migration
- Mixture of enantiomers obtained using a coordinating solvent (MeCN)
- In this case, only R migration occurs but the coordinating solvent allows solvent-aided racemization (IM lives long enough to invert before final CO addition occurs) -> most likely explanation
