Organometallic Chemistry Flashcards
What is synergic bonding?
Donation from a filled ligand to empty metal orbitals with back-donation from filled metal orbitals to vacant ligand orbitals
Liogand donates from the homo to the lumo of the metal
Metal donates to the lumo of the liagnd
This interaction can stabalise high and low oxidation states
How can organometallics influence catalysis?
Increasing the rate of reaction can occur by destabalising the reactants or stabalising the Transition state or intermediate
Transition metals can play roles in lowering the barrier heights
Cannot change the position of equilibrium just change how fast equilibrium is reached
How can addition reactions occur?
The 18 electron rule determines stability
A new two electron donor can bind to go to a 20 electron complex which can be high energy and unstable
Or it can undergo ligand loss to go to a 16 electron complex which bind a new 2 electron donor to get back to 18 electrons
What is oxidative addition?
Process where a metal reacts with an single incoming ligand to form a complex where the metal has been oxidised by 2 units and now has two new ligands:
e.g M + AB —–> M(II)(A)(B)
The sigma bond between A and B can be broken
What are the three methods the sigma bond within a ligand can be broken in oxidative addition?
Concerted oxidative addition
Heterolytic splitting
Homolytic splitting/atom transfer
What is concerted oxidative addition?
[M] + A-B —-> M(A)(B)
The oxidation state of the metal is changed by 2 units and the electron count is increased by 2
In the transition state the original ligands attached to the metal bend backwards and the bond between A and B lengthens
Sometimes the metal is not electron rich enough to break the sigma bond between A and B and there is an interaction with the bond itself to form a sigma complex
What are the activation parameters for concerted oxidative addition?
Rate of reaction = k[M][AB]
Biomolecular - creates order }+ breaks a bond
ΔH† = +ve
ΔS† = -ve
There is little charge separatipn so varying the solvent is not expected to change the barrier height significantly
A and B are always arranged cis on the metal
If A or B are chiral the product maintains the chirality
What is heterolytic splitting?
Mn + A-B —–> MnA+ + B- —-> Mn+2AB
The metal attacks the incoming group with a lone pair or is attacked by an electrophile
It is possible to stop the reaction at the first step if an 18 electron species os produced
In the first step the metal is oxidised
In the second step the -ve ligand bonds - no change in oxidation state
What are the activation parameters of heterolytic splittling?
Second order process
ΔH† = +ve
ΔS† = -ve (expected to be large)
REate faster in more polarising solvents
Inversionof stereochemistry
A and B do not have to be cis - often see mixture of trans and cis
What is homolytic splitting?
2Mn + A-B —-> Mn+1A + Mn+1B
[M] + A-B —-> M(A)(B) via M-Ao + Bo
Involves radicals - one electron goes to each centre in A-B
This reaction is less common
The radical is planar - (attack from above or below)
Loss of stereochemistry
A/B end up in any position to eacvh other on the metal
What is a migration reaction?
A reaction where an AB bond is formed by one ligand migrating onto another:
M(A)(B) —-> M(AB)
e.g: metal complex with and alkene and hydride ligand: hydride can migrate onto the alkene to form an alkane ligand.
This porcess can have polymerisation properties if it continues
What is reductive elimination?
This is the reverse reaction of oxidative addition:
Form A-B bonds from a metal complex
There are two kinds:
once centre reductive elimination
migration
What is one centre reductive elimination?
M(A)(B) —–> [M] + A-B
The barriers to this reaction can be determined by ligand design
If a 14 electron complex is formed it may decompose
If stabalising groups are added more stable metal complexes could be formed.
How can alkyl ligands be added to a metal complex?
Oxidative addition: MeI is added through hetero;ytic cleavage
Nucleophilic attack: alkyl with δ- charge can attack metal and substitute good leaving groups
e.g LiMe can replace Cl on a metal centre to form alklyl ligand and LiCl
Electrophilic attack: δ+ species can attack -vely charged ligand to neutralise charge
e.g: MeI attacks Na+[M]- to form M(Me) + NaI
Formation of salt is beneficial due to lattice energy
How can alklyl ligands decompose?
β-hydrogen elimination
α-hydrogen elimination
homolytic cleavage
What is β-hydrogen elimination?
The electrons from a C-H bond on the β carbon are donated to the metal so that the hydrogen migrated to form a hydride ligand and an alkene ligand
Alkenes can be labile and can drop off the metal complex to form a metal hydride complex
How can β-hydroge elimination be prevented?
Make the metal coordinately saturated
Make it 18 eletron
Make the ligands strong binding
Remove all β hydrogens
Make the formation of the olefin sterically or energetically unfavourable:
Bredt’s rule: double bonds to bridgehead carbons are unfavourable
What is α-hydrogen elimination?
Transfer of a hydrogen from the α-carbon to the metal resulting in the formation of a metal hydride and a carbene ligand
Less favourable than β elimination as there is poor orbital overlap
This often occurs when there is a second alkyl ligand and can lead to the elimintion of an alkene
What is homolytic cleavage?
Very rare - can be imporant in biology
LnM-CH2CH2R —-> MLn. + .CH2CH2R
Formation of radicals
What is an agostic complex?
The cleavage of the C-H bond during an elimination reaction is not always completed
The electrons of the sigma C-H bond interact with the metal but the C-H bond is not broken
The metal can gain two electrons
Can stabalise complexes with low electron counts.
How sytable are agostoc complexes?
α agostoc is known but less stable than the β agostic
What is the bonding in agostic complexes?
2 electrons donated from the sigma MO into the empty metal orbital
Back donation from a full metal orbital into the sigma * orbital
Strengthens the M-C bond
Weakens the C-H bond
How can we characterise agostic complexes?
IR - look at the C-H stretch - normal = 3000 cm-1
agostic = 2750-2350 cm-1
NMR - 1H: Normal: δ = 1-2 ppm Agistic: δ = -5 - -20 ppm
13C-H coupling: Normal: 125 Hz Agostic: 90-70 Hz
What are carbenes?
Divalent carbon compounds which in many cases are highly reactive with short lifetimes.
In stable carbenes the carbene is in a singlet state and the empty p orbital is stabalised by electron donation from other groups - create stability with large R groups
What is the synthesis of a Fischer Carbene ligand?
Assembled witin the coordination sphere of the metal
CO ligand converted to COPH ligand with LiPh so that metal has a -ve charge
[Me3O][BF4] converys the COPh to CO(Me)Ph with a formal double bond between the C and the metal
What is the bonding in carbene ligands?
In a carbene there is a full sp2 hybrid orbitak and an empty pz orbital orthogonal to it.
THere is donation from the full sp2 hybrid to the empty metal orbital
Donation fromfull metal orbital to the empty pz orbital
There can be donation from an R group on the carbene into the empty pz orbital (this only occurs in Fischer carbenes)
Differences between Fischer and Schrock carbenes?
Fischer has +M groups, Schrock does not
Fischer is a poor pi acceptor, Schrock is a good pi acceptor
Fischer is a 2 e- donor, Schrock is a 4e- donor
Schrock has more double bond character
Fischer is electrophilic, Schrock is nucleophilic
Fischer is usually formed with later transition metals, Schrock - earlier TMs
What is the chemical shift of 1H and 13C NMR of a carbene?
1H: δ 10-20 ppm
13C: δ 200-400 ppm
How is NMR used to differentiate Fischer and Schrock carbenes?
If the hydrogens on the carbon involved in the double bond are in different environments it implies there is limitted rotation and more double bond character
Carbon environments will be affected by whether or not H environments are fixed or not
What are the differences between Schrock and Grubbs single site catalysts?
Schrock - very active in swapping CR2 groups
Grubbs - Less active
Schrock: Not good with alcohols or esters
Highly water/air sensitive
High oxidation state
Grubbs: more tolerant
What are 4 types are methathesis?
Ring closing metathesis: diene —-> cyclic alkene + alkene
Cross metathesis:
alkene(R1) + alkene(R2) —-> (R1)alkene(R2) + alkene (both E+Z)
Ring opening polymeriation:
cyclic alkene — non cyclic polymer
Acyclic diene metathesis polymerisation:
diene —-> polymer + alkene
What is the Chauvin Mechanism for metathesis?
Bind an alkene to a carbene
Rearrange into 4 membered ring
Break bonds so that the original carbene is now an alkene and original alken is now the carbene
Mechanism for ring closing metathesis
What is the Dewar Chatt Duncanson bonding model for alkenes?
Donation from full π orbital to empty d orbital
Back donation from full metal orbital to empty π*
The double bind can be retained for broken by the extent of donation into the π*
What is the bonding in alkyne ligands?
Alkynes bind to ligands the same way as alkenes but have two sets of π orbitals - 2 sets of donation to the metal and backdonation
3 resonance forms
Alkynes sometimes donate more than 2 electons as a bis carbene
Why is the reversibility of the metal hydride / alkene complex useful?
Used to isomerise alkenes
Cannot turn terminal alkenes into internal alkenes
What are Vinylidenes and how can they coordinate to a metal?
high energy btautomers of alkynes
Transition metal complexes can convert alkynes to this higher energy form
The alkyne initially coordinates to the metal
Hydrogen shift gives a vinylidene
How can allyl ligands bind to a metal?
Allyl ligands can bind η1 as a 2 electron donoe or η3 as a 4 electron donor.
How can the η1 form of an allyl ligand be converted to the η3 form while bound to a metal?
Heat or light
What is the bonding in an η3 allyl ligand when binding to a metal?
How can NMR be used to show the rotation of allyl ligands?
At high temperature rotation is fast - The middle H shows as a quintet and the other two environments show up as one doublet as they can interchange
At low temps rotation is slow so a triplet of triplets is shown for the middle H and two doublets are shown for the other environments
What is a Allylic alkylation reaction?
How does catalytic allylic alkylation occur?
In catalytic allylic alkylation how can we get a enantio pure product?
Use a chiral phosphine
What are cyclobutadiene ligands?
Cyclobutadiene is an unstable molecule
Can be prepared / studied in situ by matrix isolation at 30K
Not been studied as a free species
Gains aromaticity as a ligand
Can act as an η4 ligand where all electrons are paired tp form diamagnetic species
What is the bonding between cyclobutadiene ligand and an iron complex?
What is cyclopentadianyl and why is it a useful liagand?
C5H5 = Cp
C5Me5 = Cp*
C5H4Me = Cp’
Ring has 6 pi electrons when negative so gains aromaticity
Size can protect metal
Changing substituents changes electron donation capability - stability and reactivity
What is the bonding in cyclpentadienyl ligand?
What common binding modes are there for cyclopentadienyl?
η1 and η5
In η1 the diene is retained
How is the NMR different for the η1 and η5 form of cyclopentadienyl?
The η1 should have 3 1H environments whereas the η5 should have only 1
In a complex where both forms of bonding are present at high temps 1 singlet is seen showing that η5 and η1 rings can interconvert
In reality the spectrum of an η1 complex shows broadening of the peaks - due to ring whizzing - ring bonds to metal through different carbons
What is a sandwich complex?
Originally predicted that iron could bing to 2 cyclopentadienyl ligands as η1 - This would not be possible as it would give a 10 electron complex - unstable
Instead the cyclopentadienyl liagnds bind η5 to sandwich the metal between them
The ring can rotate to give a staggered conformation with symmetry D5d or eclipsed with symmetry D5h
What is the bonding in metallocenes?
Only cyclopentadienyls of the same type can be combined
CAn be combined in the same or the opposite pahse
There is no interaction between the rings
They interact with the metal
What is the MO diagram for metallocenes?
The configuration of elefctrons within the box (e2g, a’1g and e1g* orbitals) depend on the metal and the substituents of the ring
The metal can determine whethee the elctrons are high or low spin
The substituents can add electron density which can raise the gap between t2g and eg orbitals which can promote low spin
The sandwich provoides protection to the metal which can allow higher electron counts than 18 - Co and Ni sandwiches
How can the ferrocene sandwich react?
When the loigand is bound the the metal it becomes δ- as the elecrron density moves to the metal
This can allow electrophilic substitution on the ring
The reaction does not involve the metal
Can arenes be used as ligands?
Yes - benzene ring can be used as η6 ligands
Arenes are neutral and therefore salt metathesis will not work to prepare complexes
Coordination to a metal significantly alters the properties of the arene:
If the ring has a halide substituent binding to the metal makes the ring electron rich and the promotes halide substitution
What is the bonding in an η6 arene complex?
Is it possible to make sandwich complexes using arenes?
Yes they are called Bis-arene complexes
Can larger rings coordinate to a metal?
Can larger rings make sandwich complexes?
