Organometallics Flashcards
What is hapticity?
How many atoms in the ligand are interacting with the metal
What is bridging
How many metal atoms is the ligand bridging
How to calculate O.S
Charge of complex - Charges of the ligands
How to calculate valence electrons
Group number - O.S
How to calculate TVEC
d electrons + electrons from ligands + number of M-M bonds
σ donor ligands
Have lone pair e- to donate to metal
𝜎 donor - 𝜋 acceptor ligands
Have lone pair of e- to donate to metal and also empty 𝜋 orbital to accept electrons from metal
σ donor - π donor
Have lone pair to donate as well as additional 𝜋 electrons to donate
In terms of bond strength which type is more important?
𝜎 bonding due to better orbital overlap than 𝜋 bonding
Synthesis of TM alkyls
Nucleophilic substitution of Me- group
(oxidative addition) Electrophilic attack of Me at nucleophilic metal centre
Alkene insertion into M-H bond
Reactivity of TM alkyls
β-elimination via agostic intermediate
Synthesis of TM halides
Protonation of e- rich metal centre
Reaction with a hydride (H-) source
Oxidative addition of H2 = cis dihydride
β- elimination from alkyl to form M-H bond
Reactivity of TM halides
Insertion into an alkene = alkyl chain
Reductive elimination (remove H2)
Bridging between metal centres
Synthesis of TM aryls
Nucleophilic subs of Nu Aryl group
Oxidative addition to e- rich late TM e.g. Pd
Orthometallation
Transmetallation from main group compound
Reactivity of TM carbonyls
Thermal or photochemical dissociative substitution
Addition of nucleophiles (e.g. H- or HO-)
1,1 Migratory insertion (e.g CH3)
Bonding type of CO
𝜎 donor
𝜋 acceptor
Bonding type of Phosphines
𝜎 donor
𝜋 acceptor
Synthesis of alkene complex
1) Ligand substitution
2) Reduction
3) Hydride abstraction
Reactivity of alkene TM complexes
1) 1,2 migratory insertion into M-C bond
2) Nucleophilic attack at one end of C=C bond and break C=C bond to form alkane chain instead
Alkyne TM bonding
𝜎 donation into metal vacant 𝜎 orbital
𝜋 donation into metal vacant 𝜋 orbital
𝜋 back bonding from metal into alkyne
Alkynes can be flexible in the number of electrons it donates, how many can it donate?
2 or 4
TM carbenes bonding
:C-R2
has 2 e to do 𝜎 donation to metal
and also p orbitals to do 𝜋 donation to metal
4e donor
Carbene synthesis
1) Hydride abstraction by strong cation from e.g. a methyl group to make a formal M=CH2 double bond
2) 𝛼- elimination of H onto metal to form M-H bond and M=CH2
Synthesis of Carbene TM complex from diazoalkanes
H2C=N+=N-
driving force
Loss of N2 drives reaction
CH2 binds to metal
4 types of metathesis
1) Alkene Metathesis
2) Cross metathesis (joining)
3) Ring closing metathesis
4) Ring opening metathesis polymerisation
What catalyst is used for cross metathesis
Grubbs Catalyst
Bonding mode of Allyl ligand
[C3H5]-
2 different bonding modes
1) 𝜼1 2e bonding through 1 Carbon atom
2) 𝜼3 4e bonding through 3 carbon atoms
Bonding in the cyclopentadienyl ligand
6 𝜋 electrons
-1 charge
𝜼5
𝜎 donor 𝜋 donor
𝜋 acceptor
Synthesis of Allyls
Nucleophilic attack of Metal centre to 𝛿+ Br-CH2-CH-CH2
forms 𝜼1 allyl bond through single carbon
loss of a different ligand means allyl can bond 𝜼3 through C=C
Reactivity of Allyls
1) Nucleophile can attack C-C-C neg charge delocalised area of allyl and break so only C=C bond coordinated to Metal centre rather than 𝜼3
Synthesis of cyclopentadienyls
Crack dicyclopentadiene then reduce with NaH to eliminate H2 and form (Na+)(Cp-)
Synthesis of Cp TM complexes
Reaction of NaCp with electrophilic metal centre eg with M-Cl bond
2 different bonding modes for Cp
1) 𝜼1 where electron rich metal attacks 𝛿+ carbon on Cp eg C-Br
2) 𝜼5
Reactivity of Ferrocene
Mechanisms in organometallic chemistry
1) Oxidative addition: concerted
2) Oxidative addition: SN2
3) Reductive elimination
4) 1,1 Migratory insertion
5) 1,2 Migratory insertion
Elimination Reactions
𝛼 - elimination
β - elimination
𝛾 - elimination
What is Oxidative addition: concerted
Addition of A-B to metal centre - coordinate at same time giving cis product
What is Oxidative addition: SN2
Addition of A-B to Nu- metal centre, with Metal SN2 nucleophilic attack at A+, inverting stereochemistry and kicking out B- group which then attacks pos metal centre either giving cis or trans product
Reductive elimination
2 groups must be Cis to each other, both dissociate from metal centre at same time. Often proceeded by loss of ligand to form 5 coord w flexible geometry to arrange into cis conformation
1,1 Migratory insertion
X migrates from metal centre onto C atom of CO ligand forming an acyl group and leaving vacant site on metal
1,2 migratory insertion
R (H, Alkyl or aryl) group migrates onto alkene cis to R forming an akyl chain and leaving a vacant site on metal
𝛼 - elimination
Loss of Hydride from methyl group coordinated to metal to form Carbene ligand M=CH2 and hydride ligand M-H
β - elimination
Loss of Hydride from β Carbon (2 carbons away from metal) in alkyl chain to form an alkene ligand and a hydride ligand
𝛾 - elimination
Loss of hydride from 𝛾 Carbon (3 carbons from metal) to form a alkyl 𝜼2 ligand, coordinated at both ends of chain and a hydride ligand M-H
Dissociative ligand substitution
First loss of ligand from 6 coord or 18e complex to create vacant site
Then new ligand can coordinate
Associative ligand substitution
First Coordination of new ligand onto 4 coordinate or 16e complex
Then loss of old ligand
Why is ligand substitution faster with Indenyl ligand than Cp lignand
Indenyl ligand allows ring slip to occur changing 𝜼 from 5 to 3 and electron donation from 6 to 4 so allows for associative ligand substitution which is much faster than dissociative
Bonding of Dihydrogen ligand (H2)
𝜎 donation into vacant metal orbital and 𝜋 acceptor into H-H 𝜎* antibonding orbital which weakens the H-H bond
Characterisation of H2 dihydrogen ligand upon coordinating
Dihydrogen bond length increases upon coordinating to M = weakened H-H bond
HD coupling constant decreases upon coordinating = weakened H-D bond
Frequency of HH bond decreases upon bonding = weaker H-H bond
Agostic C-H 𝜎 - complexes
e.g β-H Coordinating with metal centre weakens C-H bond
NMR coupling constant decreases
Fluxionality
Some ligands undergo dynamic processes on the NMR timescale
e.g. 𝜼5 Cp ligand rotates with all H environments are equivalent so 1 HNMR peak with integral of 5
e.g 𝜼1 Cp ligand also rotates but has 3 different proton environments.
At high temp only 1 peak of H5 seen but at lower temp see 3 different peaks for the 3 different environments
What is a catalytic cycle
Catalytic cycle is a set of sequential reactions mediated by a catalyst that leads to product formation from reactants
Alkene hydrogenation steps
1) pre catalyst dissociation of ligand (e.g. PPh3) to make vacant coord site + activate catalyst
2) CIs oxidative addition of H2
3) Coordination of the alkene
4) 1,2 migratory insertion
5) Isomerisation to move 2nd H and alkyl chain cis
6) Irreversible reductive elimination (loss of cis A and B group from metal)
Alkene Hydroformylation
1) Dissociation of ligand from pre catalyst to active
2) Coordination of propene
3) 1,2 Migratory insertion of H onto alkene
4) Coordination of CO ligand
5) 1,1 migratory insertion of alkyl onto CO
6) Cis oxidative addition of H2
7) reductive elimination
Alkene isomerisation
1) dissociation of phosphite ligand
2) Coordination of alkene
3) 1,2 migratory insertion of H onto alkene
4) β elimination of H from alkyl chain
5) Dissociation of alkene
