Metal-Carbon π-Bond (C₃, C₄, C₅) / Metallocenes

Question 1

What are the resonance forms for the following anion

Answer 1

Question 2

What are the two ways the following anion can coordinate

Answer 2

η¹ - metal acts as a 1E- donor
η³ - metal acts as a 3E- donor (all 3 carbons are interacting with the metal)

Question 3

There are 4 electrons in an allyl anion
How are they arranged between the bonding, non-bonding and anti-bonding orbtials

Answer 3

2 in the bonding
2 in the non-bonding

Question 4

Which orbitals of the metal can the bonding molecular orbitals of the allyl ion interact with?

Answer 4

any s, pz or dz² orbital
if they are in the same phase
producing a sigma-bond

Question 5

Which orbtials of the metal can the non-bonding molecular orbitals of the allyl ion interact with?

Answer 5

the py or dyz orbtials
if they are in the same phase
producing a pi-bond

Question 6

Which orbitals of the metal can the anti-bonding molecular orbitals of the allyl ion interact with

Answer 6

The px and dxz orbtials
if the are in the same phase
producing a pi bond

Question 7

What are the 3 ways we can make metal-allyl complexes

Answer 7

• From metal salts
• From allylic halides
• From alkenes

Question 8

How do metal-allyl complexes react?

Answer 8

• Nucleophilic attack
• Alkene isomerisation
• ligand displacement and insertion

Question 9

What two resonance states are possible for a metal-diene

Answer 9

RHS: diene has been reduced and metal has been oxidised

Question 10

There are 4 electrons in the diene anion
How are they arranged between the bonding, non-bonding and antibonding orbitals

Answer 10

• Two in the bonding orbital
• Two in the weakly-bonding orbitals

Question 11

What type of metal orbitals can the bonding molecular orbitals of a diene anion interact with

Answer 11

the s, pz and dz² orbitals
when all in the same phase
will form a pi-bond

Question 12

What type of metal orbitals can the weakly bonding molecular orbitals of a diene anion interact with?

Answer 12

the py and dyz orbitals
when in the same phase
will produce a pi-bonding

Question 13

What type of metal orbitals can the weakly anti-bonding molecular orbitals of a diene anion interact with?

Answer 13

the px and dxz orbitals
when in the same phase
will produce pi bonding (back-donation from metal to ligand)

Question 14

What type of metal orbital can the anti-bonding molecular orbtials of a diene anion interact with?

Answer 14

the dxy orbitals
producing a δ-backbonding interaction

Question 15

cyclobutadiene has 4 pi-electrons
What is the arrangement of these electrons according to Huckel’s rules
Why in reality is this not the case if it is not a ligand?

Answer 15

• It is expected to be antiaromatic, diradial (two unpaired electrons) as predicted by Huckel’s rules - sqaure shaped
• The free molecule (not as a ligand) of cyclobutadiene in reality distorts to a rectangle, allowing two localised double and single bonds (no-longer degenerate)

Question 16

What type of metal orbital can the anti-bonding molecular orbitals of cyclobutadiene interact with

Answer 16

the dx²-dy²
Forming δ-backbond interaction

Question 17

How can you can synthesis a metal-diene complex?

Answer 17

• ligand substitution of other 2e- donors

Question 18

What is the way you can synthesis a metal-cyclobutadiene complex?

Answer 18

Through make the cyclobutadiene in situ
e.g. perhaps through a reduction of 4 membered rings called dehalogenation
alkene dimerisation
or ring contraction

Question 19

What are the ways in which dienes/cyclobutadienes can react?

Answer 19

• Ligand substitution (diene quiet easy to remove)
• Diene isomerisation
• Electrophilic ring substitution

Question 20

Cp is the abbreviation for

Answer 20

C₅H₅⁻

Question 21

How many pi-electrons does the Cp anion have

Answer 21

6 pi-electrons

Question 22

What possible co-ordination states are there for the Cp ligand

Answer 22

usually η⁵ (3 coordination sites)
but also η¹ and η³

Question 23

What is the CP*
And what is special about it

Answer 23

Is a much stronger electron donor than Cp (and a poorer electron acceptor) so complexes with this ligand have their redox potential shifted to more negative values (i.e. easier to oxidise, more reducing)

Question 24

How can you form cyclopentadienyl anion from cyclopentadiene?

Answer 24

By reacting cyclopentadiene with a strong base (e.g. NaH)

Question 25

Cyclopentadiene cannot be bought as a pure substance How can it be synthesised?

Answer 25

Through heating a cyclopentadiene dimer causing it to undergo a retro-diels alder reaction

Question 26

Sodium Cp can then react with metals salts to form a metallocene through...

Answer 26

salt elimination reaction This reaction forms ferrocene - iron with 2x Cp ligands

Question 27

what orbital arrangements are possible for a metallocene ligand

Answer 27

* 5 p-orbitals combine to give 5 molecular orbitals, with a radical * bonding arragement (all in phase) * weakly bonding (1 nodal plane) * antibonding (2 nodal planes)

Question 28

What type of bonding occurs for the following orbital arrangement of the metallocene ligand?

Answer 28

sigma bonding

Question 29

What type of bonding occurs for the following orbital arrangement of the metallocene ligand?

Answer 29

one nodal plane which can interact with an pi-type orbital also with a nodal plane forming a pi-bond (mostly a pi-donor interaction)

Question 30

What type of bonding occurs for the following orbital arrangement of the metallocene ligand?

Answer 30

these two anti-bonding orbitals have two nodal planes This allows for a δ-type backbonding interaction (weak interaction due to poor overlap)

Question 31

Why it is possible to generate a whole range of different electron count with metallocenes

Answer 31

There is very little difference in energy between the weakly bonding and weakly antibonding orbitals (blue arrows) Due to the 5-C pi-orbitals not aligning very well Hence due to the little energy difference it is not as unfavourable to add or take away an electron There metallocene tend not to obey the 18 electron rule as much (18e- complex is still the most stable)

Question 32

What are some metallocene which execption to not following the 18e- rule?

Answer 32

Titanium and Cobalt Due to electron deficiency

Question 33

What is a para-metallocene?

Answer 33

a metal which has two Cp groups above and below it

Question 34

What is a bent metallocene?

Answer 34

Bending a metallocene raises its energy (HOMO and LUMO are the metal not the ligand), but this is offset by the energetic stabilisation gained from forming additional M-L bonds Bent metallocenes will react at the metal centre - rather than the ring

Question 35

Bending of the metallocene results in?

Answer 35

* Destructive interference of orbitals where the ring approaches resulting in an asymmetry of the electron distribtution * This means that bent metallocenes complexes have three frontier orbitals available for bonding to ligands and substrates

Question 36

How does the 18e- rule affect metallocene complexes?

Answer 36

The 18e- rule means that bent metallocene complexes are restricted to low dₙ metals, or ring slippage (η⁵→η³→η¹) may occur The early transition metals are likely not to have an 18e- count

Question 37

What is the product of this react and the benefit of undertaking it?

Answer 37

Form metallocene dihalides which are an important class of compounds, synthetically and for their in vitro activity against cancer cells Reactions include: alkylations, reduction, exchange reactions with donor ligands