ME - Main Group Organometallics
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81 Terms
What is an organometallic compound?
A compound containing at least one direct metal–carbon (M–C) bond.
Why are organometallic compounds usually reactive?
The M–C bond is polarised (Mδ+–Cδ−)
What is transmetallation?
Transfer of an organic group from one metal to another.
General transmetallation reaction
RLi + MXn → RM + LiX.
What is protonolysis?
Reaction of an organometallic compound with a proton source such as water
What happens when organometallics react with water?
The M–C bond is destroyed and a hydrocarbon (RH) is formed.
Example of protonolysis
CH3Li + H2O → CH4 + LiOH.
What is hydrometallation?
Addition of an M–H bond across a C=C double bond.
What type of mechanism is involved in hydrometallation?
A concerted four-centre transition state.
Name four hydrometallation reactions
Hydroboration; hydrosilylation; hydroalumination; hydrostannylation.
Why are organolithium compounds highly reactive?
The Li–C bond is highly polar and ionic.
Give three examples of organolithium reagents
MeLi; n-BuLi; PhLi.
How are organolithium compounds prepared?
RX + 2Li → RLi + LiX.
What conditions are required for preparing organolithiums?
Dry solvents and an inert atmosphere.
What is halogen–metal exchange?
Exchange of a halogen atom with lithium to form a new organolithium reagent.
Why do organolithium compounds aggregate?
Lithium is electron deficient and seeks additional stabilising interactions.
What aggregates can organolithiums form?
Dimers; tetramers; hexamers.
How do donor solvents affect organolithium aggregation?
They coordinate to Li+ and reduce aggregation.
Examples of donor solvents used with organolithiums
THF and diethyl ether.
How does reactivity change down Group 1 organometallics?
Li < Na < K.
Why are organopotassium compounds more reactive than organolithiums?
The K–C bond is more ionic.
How is sodium cyclopentadienide prepared?
CpH + NaH → NaCp + H2.
Why is the cyclopentadienyl anion important?
It stabilises many transition-metal complexes.
What is a Grignard reagent?
An organomagnesium compound of formula RMgX.
How are Grignard reagents prepared?
RX + Mg → RMgX.
Which solvents are commonly used for Grignard reagents?
Ether or THF.
Why must Grignard reactions be performed under dry conditions?
Water destroys the reagent by protonolysis.
What is the Schlenk equilibrium?
2RMgX ⇌ R2Mg + MgX2.
Why is the Schlenk equilibrium important?
Multiple magnesium-containing species exist in solution.
What species may be present in a Grignard solution?
RMgX; R2Mg; MgX2; solvent adducts.
What happens when Grignard reagents react with water?
RMgX + H2O → RH.
What is an important synthetic application of Grignard reagents?
Addition to aldehydes and ketones to form alcohols.
How are organoboron compounds commonly prepared?
From boron halides or by hydroboration.
What is hydroboration?
Addition of BH3 across a C=C bond.
What stereochemistry does hydroboration give?
Syn addition.
What regioselectivity does hydroboration give?
Anti-Markovnikov addition.
Why is hydroboration anti-Markovnikov?
Boron attaches to the less substituted carbon in the concerted transition state.
Does hydroboration proceed through a carbocation intermediate?
No.
What is the mechanism of hydroboration?
A concerted four-centre transition state.
How can trialkylboranes be prepared?
BCl3 + 3RLi → BR3 + 3LiCl.
Give two examples of organoaluminium compounds
AlEt3 and AlMe3.
In what form does AlMe3 usually exist?
As the dimer Al2Me6.
What type of bonding occurs in Al2Me6?
Three-centre two-electron Al–C–Al bonding.
Why does Al2Me6 form?
Aluminium is electron deficient.
What is the Aufbau (Ziegler growth) reaction?
Successive insertion of ethene into Al–C bonds.
What is the key step in the Aufbau reaction?
Alkene insertion into an Al–C bond.
Industrial importance of the Aufbau reaction
Production of long-chain alkyl compounds and polymers.
How are organosilicon compounds prepared from SiCl4?
SiCl4 + 4RLi → SiR4 + 4LiCl.
What is the industrial route to methylchlorosilanes?
Reaction of MeCl with Si over a Cu catalyst.
What is hydrosilylation?
Addition of Si–H across a C=C bond.
Applications of hydrosilylation
Silicone manufacture and surface modification.
What is the repeating unit in silicones?
–SiR2–O–.
Key properties of silicones
Flexible; heat-resistant; water-resistant.
What oxidation state is most common in organogermanium compounds?
Ge(IV).
Why are Ge(II) compounds less common?
They are less stable and require bulky ligands.
What is the Lappert strategy?
Using bulky ligands to stabilise low oxidation-state compounds.
How are organotin compounds prepared?
SnCl4 + 4RLi → SnR4 + 4LiCl.
What is hypervalency?
An atom having more than eight electrons in its valence shell.
Why can tin form hypervalent compounds?
It can accommodate higher coordination numbers than carbon.
What coordination numbers are common for hypervalent tin compounds?
5 and 6.
What geometries are common for hypervalent tin compounds?
Trigonal bipyramidal and octahedral.
What is tetraethyllead?
PbEt4
Why was tetraethyllead added to petrol?
To improve octane rating and reduce engine knocking.
Why was tetraethyllead phased out?
Lead is highly toxic and causes environmental contamination.
What are the common Group 15 organometallic ligands?
Phosphines; arsines; stibines; bismuthines.
General formula for triorganopnictides
ER3 (E = P
Why are phosphines important?
They are widely used ligands in transition-metal chemistry.
What geometry is common for E(V) organometallic compounds?
Trigonal bipyramidal.
What are the two positions in a trigonal bipyramidal structure?
Axial and equatorial.
What is Berry pseudorotation?
Rapid exchange of axial and equatorial substituents in trigonal bipyramidal molecules.
How does Berry pseudorotation affect NMR spectra?
All equivalent groups can give a single averaged signal.
Why do all methyl groups appear equivalent in SbMe5 NMR?
Rapid Berry pseudorotation averages environments.
What happens when triorganopnictides are exposed to oxygen?
They oxidise to R3E=O compounds.
Why are small trialkyl pnictides often pyrophoric?
They react readily with oxygen in air.
Are triaryl pnictides more or less stable than trialkyl pnictides?
More stable.
What is the Tolman cone angle?
A measure of the steric bulk of a ligand.
Why is the Tolman cone angle important?
It influences metal complex reactivity and structure.
How can ligand properties be tuned?
By changing the steric and electronic properties of the substituents.
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What are the highest-yield structures to memorise?
Organolithium aggregates; Al2Me6; hypervalent organotin compounds; trigonal bipyramidal pnictides.
What are the highest-yield concepts to explain in exams?
M–C bond polarity; aggregation of organolithiums; Schlenk equilibrium; anti-Markovnikov hydroboration; Berry pseudorotation; hypervalency.