Metal-Carbonyls/Metal-Carbon π-bonds Flashcards
How does the bonding occur between a metal-carbonyl complex?
- The HOMO is CO is the sσ’ orbital containing a pair of E-, which are donated to the metal (sigma donation)
- Empty π’ orbitals can reciece E- from the metal through π-back donation - this affect dominates
- (known as synergeric bonding which results in a strengthening of the M-C bond)
What affect does the π-back donation have on the strength of the C-O bond in a metal-carbonyl complex
Weakens the C-O
This will result in a longer C-O and a decreased vCO on the IR spectra
the more electron rich the metal is in the metal carbonyl complex, the ……. stable the metal-carbonyl complex is
more stable
The more π-back donation can occur, which stabilises the CO bound to the metal
What happens to the vibrating frequency of the C-O when bonded to a metal?
- The vibrating frequency decreases with the more M-CO bonds added
- Does depend on the metal due to more electro rich metals π-back donating, resulting in weaker M-CO bonds
- The H₃B-CO shows the effect of just sigma donation = higher frequency vibrations
What happens to the IR vibration frequency of the C-O, when the bonded metal-carbonyl moves to a lower oxidation state
The lower the oxidation state, will mean they are electron rich and hence will π-back donate more
Resulting in weaker C-O and hence lower frequency stretches
What happens to the IR vibration frequency of the C-O when electron donating ligands are also bonded to the metal?
The more electron donating the other ligands are, the lower frequency vibration of the C-O there is
What is one of the simplest ways to make a metal-carbonyl
Through reacting the metal with carbon monoxide gas
What is another way you can make a metal-carbonyl
- Through using a metal halide and reduce (Na) it to a lower oxidation state, which will make it favourable for CO to bind
What is two ways to remove a CO ligands from a metal?
- CO is easily eliminated by heat or photolysis
CO can be replaced in a metal complex by
Other neutral, two electron donors
What type of reaction is this?
oxidation
Goes to a Mn +1 when we formed the manganese iodide species
(both species are 18 electrons)
Why is reduction a more common process for metal-carbonyl species
The carbonyl species are good at withdrawing electron density through the pi-system
So they stabilise metals in low oxidation states i.e. stabilise metals with a lot of electron density
How would you reduce a metal-carbonly species?
- Either: by reduction using sodium
- OR: Nu attack using sodium hydroxides which will attack the CO ligand
What is the end product of this reaction?
- R group will form a new bond to one of the carbonyls
(1,1-migratory insertion) - The negative charge on the metal makes it nucleophilic for this reaction to occur
How does the bonding occur in a Metal-Alkene complex?
- The Alkene donates to the metal through the HOMO π-bonding orbital
- The alkene accepts π-back-donation into the LUMO π’ antibonding orbital
- (synergic again- the two components (back and forward donation) of the bonding are better balanced than the metal-carbonyl complexes)
What affect does the metal-alkene bonding have on the bonding in the alkene
- Lengthening of the C=C bond - as it is weaker due to the back donation
- Reduction of angles at C from 120° (sp²) to 109° (sp³) if there is enough back donation due to the breaking of the C=C
What does the extent of backbonding depend on?
- The energy of the frontier orbital of the M fragment
- Steric effects
- The alkene acceptor ability
Electron withdrawing group affect π-back donation how?
Electron withdrawing groups increase π-backdonation and decrease σ-donation
Alkene ligands can be switched in with which type of ligands?
- Switched for other neutral two electron donors
How do metal-alkene complexes react with nucleophiles
- Through a 1,2-migratory insertion
- This is due to so much electron density can be donated from the alkene to the metal which results in it being positively charged - hence more susceptible to Nu attack
True or false?
Alkynes are weaker π-acceptors than alkenes
False
Alkynes are stronger π-acceptors than alkenes
Alkynes can donate how many electrons as a ligand?
Alkynes have two orthogonal π-bonds and can act as 2 or 4 electron donor ligands
Significant amounts of back donation from the metal, can result in an alkynes being
sp² hybridised rather than sp hybridised
notes the longer bond too
Alkyne ligands can be swithced in/out by which type of ligand?
- Neutral 2 or 4 electron ligands
Usually triple bonds with benzene are unstable
What allows this compound to exist?
Through binding to Zirconium it is closer to being sp² hybridised (back donation) hence is more stable
What it the product of this reaction
Formation of an aromatic ring through combining the alkene with CO groups
What is the product of this reaction?
through a series of oxidative couplings
