Metal-Carbonyls/Metal-Carbon π-bonds

Question 1

How does the bonding occur between a metal-carbonyl complex?

Answer 1

• 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)

Question 2

What affect does the π-back donation have on the strength of the C-O bond in a metal-carbonyl complex

Answer 2

Weakens the C-O
This will result in a longer C-O and a decreased vCO on the IR spectra

Question 3

the more electron rich the metal is in the metal carbonyl complex, the ……. stable the metal-carbonyl complex is

Answer 3

more stable
The more π-back donation can occur, which stabilises the CO bound to the metal

Question 4

What happens to the vibrating frequency of the C-O when bonded to a metal?

Answer 4

• 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

Question 5

What happens to the IR vibration frequency of the C-O, when the bonded metal-carbonyl moves to a lower oxidation state

Answer 5

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

Question 6

What happens to the IR vibration frequency of the C-O when electron donating ligands are also bonded to the metal?

Answer 6

The more electron donating the other ligands are, the lower frequency vibration of the C-O there is

Question 7

What is one of the simplest ways to make a metal-carbonyl

Answer 7

Through reacting the metal with carbon monoxide gas

Question 8

What is another way you can make a metal-carbonyl

Answer 8

• Through using a metal halide and reduce (Na) it to a lower oxidation state, which will make it favourable for CO to bind

Question 9

What is two ways to remove a CO ligands from a metal?

Answer 9

• CO is easily eliminated by heat or photolysis

Question 10

CO can be replaced in a metal complex by

Answer 10

Other neutral, two electron donors

Question 11

What type of reaction is this?

Answer 11

oxidation
Goes to a Mn +1 when we formed the manganese iodide species
(both species are 18 electrons)

Question 12

Why is reduction a more common process for metal-carbonyl species

Answer 12

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

Question 13

How would you reduce a metal-carbonly species?

Answer 13

• Either: by reduction using sodium
• OR: Nu attack using sodium hydroxides which will attack the CO ligand

Question 14

What is the end product of this reaction?

Answer 14

• 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

Question 15

How does the bonding occur in a Metal-Alkene complex?

Answer 15

• 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)

Question 16

What affect does the metal-alkene bonding have on the bonding in the alkene

Answer 16

• 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

Question 17

What does the extent of backbonding depend on?

Answer 17

• The energy of the frontier orbital of the M fragment
• Steric effects
• The alkene acceptor ability

Question 18

Electron withdrawing group affect π-back donation how?

Answer 18

Electron withdrawing groups increase π-backdonation and decrease σ-donation

Question 19

Alkene ligands can be switched in with which type of ligands?

Answer 19

• Switched for other neutral two electron donors

Question 20

How do metal-alkene complexes react with nucleophiles

Answer 20

• 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

Question 21

True or false?
Alkynes are weaker π-acceptors than alkenes

Answer 21

False
Alkynes are stronger π-acceptors than alkenes

Question 22

Alkynes can donate how many electrons as a ligand?

Answer 22

Alkynes have two orthogonal π-bonds and can act as 2 or 4 electron donor ligands

Question 23

Significant amounts of back donation from the metal, can result in an alkynes being

Answer 23

sp² hybridised rather than sp hybridised
notes the longer bond too

Question 24

Alkyne ligands can be swithced in/out by which type of ligand?

Answer 24

• Neutral 2 or 4 electron ligands

Question 25

Usually triple bonds with benzene are unstable
What allows this compound to exist?

Answer 25

Through binding to Zirconium it is closer to being sp² hybridised (back donation) hence is more stable

Question 26

What it the product of this reaction

Answer 26

Formation of an aromatic ring through combining the alkene with CO groups

Question 27

What is the product of this reaction?

Answer 27

through a series of oxidative couplings