Metal-Carbon π-Bond (C₃, C₄, C₅) / Metallocenes Flashcards
What are the resonance forms for the following anion
What are the two ways the following anion can coordinate
η¹ - metal acts as a 1E- donor
η³ - metal acts as a 3E- donor (all 3 carbons are interacting with the metal)
There are 4 electrons in an allyl anion
How are they arranged between the bonding, non-bonding and anti-bonding orbtials
2 in the bonding
2 in the non-bonding
Which orbitals of the metal can the bonding molecular orbitals of the allyl ion interact with?
any s, pz or dz² orbital
if they are in the same phase
producing a sigma-bond
Which orbtials of the metal can the non-bonding molecular orbitals of the allyl ion interact with?
the py or dyz orbtials
if they are in the same phase
producing a pi-bond
Which orbitals of the metal can the anti-bonding molecular orbitals of the allyl ion interact with
The px and dxz orbtials
if the are in the same phase
producing a pi bond
What are the 3 ways we can make metal-allyl complexes
- From metal salts
- From allylic halides
- From alkenes
How do metal-allyl complexes react?
- Nucleophilic attack
- Alkene isomerisation
- ligand displacement and insertion
What two resonance states are possible for a metal-diene
RHS: diene has been reduced and metal has been oxidised
There are 4 electrons in the diene anion
How are they arranged between the bonding, non-bonding and antibonding orbitals
- Two in the bonding orbital
- Two in the weakly-bonding orbitals
What type of metal orbitals can the bonding molecular orbitals of a diene anion interact with
the s, pz and dz² orbitals
when all in the same phase
will form a pi-bond
What type of metal orbitals can the weakly bonding molecular orbitals of a diene anion interact with?
the py and dyz orbitals
when in the same phase
will produce a pi-bonding
What type of metal orbitals can the weakly anti-bonding molecular orbitals of a diene anion interact with?
the px and dxz orbitals
when in the same phase
will produce pi bonding (back-donation from metal to ligand)
What type of metal orbital can the anti-bonding molecular orbtials of a diene anion interact with?
the dxy orbitals
producing a δ-backbonding interaction
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?
- 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)
What type of metal orbital can the anti-bonding molecular orbitals of cyclobutadiene interact with
the dx²-dy²
Forming δ-backbond interaction
How can you can synthesis a metal-diene complex?
- ligand substitution of other 2e- donors
What is the way you can synthesis a metal-cyclobutadiene complex?
Through make the cyclobutadiene in situ
e.g. perhaps through a reduction of 4 membered rings called dehalogenation
alkene dimerisation
or ring contraction
What are the ways in which dienes/cyclobutadienes can react?
- Ligand substitution (diene quiet easy to remove)
- Diene isomerisation
- Electrophilic ring substitution
Cp is the abbreviation for
C₅H₅⁻
How many pi-electrons does the Cp anion have
6 pi-electrons
What possible co-ordination states are there for the Cp ligand
usually η⁵ (3 coordination sites)
but also η¹ and η³
What is the CP*
And what is special about it
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)
How can you form cyclopentadienyl anion from cyclopentadiene?
By reacting cyclopentadiene with a strong base (e.g. NaH)
Cyclopentadiene cannot be bought as a pure substance
How can it be synthesised?
Through heating a cyclopentadiene dimer
causing it to undergo a retro-diels alder reaction
Sodium Cp can then react with metals salts to form a metallocene through…
salt elimination reaction
This reaction forms ferrocene - iron with 2x Cp ligands
what orbital arrangements are possible for a metallocene ligand
- 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)
What type of bonding occurs for the following orbital arrangement of the metallocene ligand?
sigma bonding
What type of bonding occurs for the following orbital arrangement of the metallocene ligand?
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)
What type of bonding occurs for the following orbital arrangement of the metallocene ligand?
these two anti-bonding orbitals have two nodal planes
This allows for a δ-type backbonding interaction (weak interaction due to poor overlap)
Why it is possible to generate a whole range of different electron count with metallocenes
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)
What is a para-metallocene?
a metal which has two Cp groups above and below it
Bending of the metallocene results in?
- 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
How does the 18e- rule affect metallocene complexes?
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
What is the product of this react and the benefit of undertaking it?
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
