Organometallic Chemistry Flashcards
Alkene
any of the series of unsaturated hydrocarbons containing a double bond, including ethylene and propene.
Alkyne
any of the series of unsaturated hydrocarbons containing a triple bond, including acetylene
Allyl
of or denoting the unsaturated hydrocarbon radical —CH2CH=CH2.
Arene
a hydrocarbon with alternating double and single bonds between carbon atoms forming rings
Back bonding
a concept from chemistry in which electrons move from an atomic orbital on one atom to an appropriate symmetry antibonding orbital on a π-acceptor ligand
β-hydride elimination
a reaction in which an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene
β-hydrogen transfer
the opposite of beta-hydride elimination
Bridging ligand
a ligand that connects two or more atoms, usually metal ions
Carbonyl
a coordination compound in which one or more carbon monoxide molecules are bonded as neutral ligands to a central metal atom.
Cyclopentadienyl
[C5H5]
EAN rule
the metal atom tends to surround itself with sufficient ligands that the resulting effective atomic number is numerically equal to the atomic number of the noble-gas element found in the same period in which the metal. Also known as the 18 electron rule
Ferrocene
an orange crystalline compound whose molecule has a sandwich structure in which two planar cyclic hydrocarbon ligands enclose an iron atom
Hapticity
the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η (‘eta’)
HOMO
a molecular orbital characterised by the highest occupied molecular orbital
Homogenous catalysis
catalytic reactions where the catalyst is in the same phase as the reactants
Homolytic fission
chemical bond dissociation of a molecule by a process where each of the fragments retains one of the originally bonded electrons
Hydroformylation
an industrial process for the production of aldehydes from alkenes
Infrared spectroscopy
a characterization tool chemists use to help determine the molecular structure. It capitalizes on the concept that functional groups absorb specific frequencies of energy based on their structure
Insertion reactions
a chemical reaction where one chemical entity (a molecule or molecular fragment) interposes itself into an existing bond of typically a second chemical entity
LUMO
the lowest energy place to put or excite an electron
Metallation
a chemical reaction which involves the bonding of a metal atom to what is typically an organic molecule to form a new compound
Nucleophilic attack
a fundamental class of reactions in which an electron rich nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms to replace a leaving group
Organometallic compound
any member of a class of substances containing at least one metal-to-carbon bond in which the carbon is part of an organic group
Phosphine ligand
compound of the formula PRR’R” (R, R’, R” = H, alkyl, aryl, etc) that are used as ligands in metal complexes
π-acid ligand
a molecule that binds to a metal by accepting electrons through (antibonding) pi-orbitals
Redox transmetallation
a type of organometallic reaction that involves the transfer of ligands from one metal to another
Reductive elimination
an eliminatory step in organometallic chemistry in which the oxidation state of the metal center decreases while forming a new covalent bond between two ligands
σ-complex
The positively charged intermediate of an electrophilic aromatic substitution reaction
Synergism
An effect arising between two or more agents, entities, factors, or substances that produces an effect greater than the sum of their individual effects.
Define
Terminal ligand
Definition
any of the series of unsaturated hydrocarbons containing a double bond, including ethylene and propene.
Alkene
Definition
any of the series of unsaturated hydrocarbons containing a triple bond, including acetylene
Alkyne
Definition
of or denoting the unsaturated hydrocarbon radical —CH2CH=CH2.
Allyl
Definition
a hydrocarbon with alternating double and single bonds between carbon atoms forming rings
Arene
Definition
a concept from chemistry in which electrons move from an atomic orbital on one atom to an appropriate symmetry antibonding orbital on a π-acceptor ligand
Back bonding
Definition
a reaction in which an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene
β-hydride elimination
Definition
the opposite of beta-hydride elimination
β-hydrogen transfer
Definition
a ligand that connects two or more atoms, usually metal ions
Bridging ligand
Definition
a coordination compound in which one or more carbon monoxide molecules are bonded as neutral ligands to a central metal atom.
Carbonyl
Definition
[C5H5]
Cyclopentadienyl
Definition
the metal atom tends to surround itself with sufficient ligands that the resulting effective atomic number is numerically equal to the atomic number of the noble-gas element found in the same period in which the metal. Also known as the 18 electron rule
EAN rule
Definition
an orange crystalline compound whose molecule has a sandwich structure in which two planar cyclic hydrocarbon ligands enclose an iron atom
Ferrocene
Definition
the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η (‘eta’)
Hapticity
Definition
a molecular orbital characterised by the highest occupied molecular orbital
HOMO
Definition
catalytic reactions where the catalyst is in the same phase as the reactants
Homogenous catalysis
Definition
chemical bond dissociation of a molecule by a process where each of the fragments retains one of the originally bonded electrons
Homolytic fission
Definition
an industrial process for the production of aldehydes from alkenes
Hydroformylation
Definition
a characterization tool chemists use to help determine the molecular structure. It capitalizes on the concept that functional groups absorb specific frequencies of energy based on their structure
Infrared spectroscopy
Definition
a chemical reaction where one chemical entity (a molecule or molecular fragment) interposes itself into an existing bond of typically a second chemical entity
Insertion reactions
Definition
the lowest energy place to put or excite an electron
LUMO
Definition
a chemical reaction which involves the bonding of a metal atom to what is typically an organic molecule to form a new compound
Metallation
Definition
a fundamental class of reactions in which an electron rich nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms to replace a leaving group
Nucleophilic attack
Definition
any member of a class of substances containing at least one metal-to-carbon bond in which the carbon is part of an organic group
Organometallic compound
Definition
compound of the formula PRR’R” (R, R’, R” = H, alkyl, aryl, etc) that are used as ligands in metal complexes
Phosphine ligand
Definition
a molecule that binds to a metal by accepting electrons through (antibonding) pi-orbitals
π-acid ligand
Definition
a type of organometallic reaction that involves the transfer of ligands from one metal to another
Redox transmetallation
Definition
an eliminatory step in organometallic chemistry in which the oxidation state of the metal center decreases while forming a new covalent bond between two ligands
Reductive elimination
Definition
The positively charged intermediate of an electrophilic aromatic substitution reaction
σ-complex
Definition
An effect arising between two or more agents, entities, factors, or substances that produces an effect greater than the sum of their individual effects.
Synergism
Terminal ligand
Is Na4(CH3)4 an organometallic compound?
Yes
Is N(CH3)3 an organometallic compound?
No
Is CH3COONa an organometallic compound?
No
Is [W(CO)6] an organometallic compound?
Yes
What are the property differences between coordination and organometallic compunds?
Coordination compounds
- Water soluble
- Not air sensitive
- High melting points (>250OC)
Organometallic compounds
- Hydrocarbon soluble
- Air sensitive
- Low melting points or oils
True or False:
CO is an important pi-base ligand
False
It is a pi-acid ligand
a) Valence bond structure of CO shows ______ non-bonding electron pairs
b) Which electrons are the most available for donation to the metal on a CO
a) two
b) the electron pair on the carbon
When is back bonding possible?
This type of bonding is possible between atoms in a compound in which one atom has lone pair of electron and the other has vacant orbital placed adjacent to each other.
Back bonding results in a ________in bond length and ______-in bond order giving it partial double bond character
Back bonding results in a decrease in bond length and increase in bond order giving it partial double bond character
What does the electron density in an anti-bonding orbital do to the C bond during back bonding?
Weakens the bond
What happens during back bonding?
the electrons move from an atomic orbital on one atom to a π* anti-bonding orbital on another atom or ligand
CO ligands are sigma _______ and pi _________
CO ligands are sigma donors and pi acceptors
CO has much less of a dipole moment than expected. Explain this using your knowledge of back bonding
CO has much less dipole moment than expected due to back-donation from LONE PAIR ORBITAL OF Oxygen into vacant orbital of carbon
BF3 < BCl3 < BBr3 < BI3 is order of lewis acidic character. Explain this observation using back bonding
BF3 < BCl3 < BBr3 < BI3 is order of lewis acidic character due to stronger 2pπ-2pπ back bonding in BF3 (LONE PAIR ORBITAL OF Fluorine into vacant orbital of boron) and gradually back bonding becomes weakest in BI3 (2pπ—5pπ) hence BF3 has stronger partial double bond character and consequently behaves as less electron deficient
As the strength of a sigma bond increases, what happens to the back bond?
It also increases
Is ligand-to-metal or metal-to-ligand bonding more energetically important?
Ligand-to-metal
Why are some metal compounds stable in a 0 oxidation state?
Backbonding
What are the two methods used to synthesise metal carbonyls?
Direct reaction
Reductive carbonylation
M + nCO → [M(CO)n]
What is this type of reaction called?
Direct reaction
What metals can undergo the direct reaction to synthesise metal carbonyls?
Iron (Fe) and Nickel (Ni)
“M” + CO + reductant → [M(CO)n]
“M” = metal halide, metal oxide etc.
What is this type of reaction?
Reductive carbonylation synthesis of metal carbonyls
How many electrons does a C5H5- ligand donate?
6
How many electrons does a terminal CO donate?
2
How many electrons does a bridging carbonyl donate to each metal?
1
Does [Ni(CO)4] satisfy the 18-electron rule?
Yes
Predict the formula of [Fe(CO)n]
n = 5
What does the Greek symbol η indicate?
Hapticity or the number of carbons bonded to metal
Explain how [Fe2(CO)9] has 18 electrons around each Fe?
What usually happens to complexes with 16e- or 14e-?
They are very reactive and will try and increase their coordination number
What usually happens to 17e- compounds?
They will dimerise or form an anion to complete 18e-
What are these reactions an example of?
Substitution reactions
[Mn2(CO)10] + Cl2 →
[Mn2(CO)10] + Cl2 → 2 [Mn(CO)5Cl]
[Fe(CO)5] + Br2 →
[Fe(CO)5] + Br2 → [Fe(CO)4Br2] + CO
[Mo(CO)6] + excess MeCN →
[Mo(CO)6] + excess MeCN → [Mo(CO)3(NCMe)3] + 3CO
[Mn2(CO)10] + H2 →
[Mn2(CO)10] + H2 → 2 [Mn(CO)5H]
PIII ligands have a lone pair for ___-donation, as well as empty d-orbitals of ___-symmetry
PIII ligands have a lone pair for σ-donation, as well as empty d-orbitals of π-symmetry
π-acidity of phosphine ligands is enhanced by what?
Electron withdrawing groups
True or False:
Back bonding occurs for phosphine ligands
True
List these ligands in order of increasing Tolan cone angle:
PPh3, PF3, P(tBu)3, P(CH3)3
PF3 < P(CH3)3 < PPh3 < P(tBu)3
If the highly substituted complex [Mo(CO)3L3] is desired, which of the ligands P(CH3)3 or P(tBu)3 would be preferred?
P(CH3)3
Smaller Tolman cone angle
What does the number of IR absorption bands of metal carbonyls relate to?
Symmetry
What does an increase in back bonding do to IR frequency?
It lowers the wavenumber due to weaker bonding
Predict which complex will exhibit the lower IR CO stretching frequency:
[Cr(CO)5(PEt3)] and [Cr(CO)5(PPh3)]
[Cr(CO)5(PEt3)]
It is a better Lewis base
Ethyl groups are more e- donating than phenyl groups
PEt3 is smaller than PPh3
Predict which complex will exhibit the lower IR CO stretching frequency:
[Cr(CO)6] and [V(CO)6]-
[V(CO)6]-
Anion orbitals on metal center are bigger allowing more overlap with π* ligand orbitals
How many electrons can an ethene ligand donate?
2
How many electrons can cyclopentadienyl ligands donate?
6
How many electrons can benzene ligands donate?
6
What are the resonance structures of cyclopentadienyl?
Why is ferrocene stable?
The sandwich conformation makes it hard for molecules to interact with the metal center
Predict whether the following complex is stable:
[CrCp2]
Both stable and unstable are correct answers as long as explanations are valid:
Stable: due to sandwich conformation protecting the metal center
Unstable: due to 16e- complex, reactive when exposed to oxygen
What ligand is Cp?
Cyclopentadienyl; [C5H5]-
Draw the structure of [CpFe(CO)2]2
Demonstrate that this complex satisfies the 18e rule
Predict the product(s) of this reaction:
[CpFe(CO2)2 + Br2 →
[CpFe(CO2)2 + Br2 → 2[CpFe(CO)2Br] (oxidation)
Why does ethene lose it planarity and its C-C bond lengths upon coordination to a Pt center?
Moves towards sp3 hybridisation
How many sigma bonds is in a Zeise’s salt molecule?
8
The bond between the ethene and the Pt is not a pi or sigma bond due to back bonding effects
Explain the bonding that occurs in Ziese’s salt
For the formation of bond between the metal and the ligand, electrons from filled π-bonding MO of ethene (πb) is donated to the vacant orbitals of Platinum. And electrons from the filled orbitals of Pt is back-donated to the vacant π∗ orbitals of ethene.
Why are arenes more susceptible to deprotonation and nucleophilic substitution than free benzene?
Because electron density is sucked away from the coordinated benzene ring making it vulnerable to nucleophilic attack
Why do alkynes tend to bind more tightly to transition metals than alkenes?
Because they are more more electropositive
How many electrons does an alkyne ligand donate?
2 or 4
How does alkyne donate 2 electron?
Only one out of 2 of its pi bond is involved
Allyl ligands are sigma _______
Allyl ligands are sigma donors
How many electrons is donated by allyl in this form?
2
How many electrons are donated by allyl in this form?
4
What is the reverse reaction of a β-hydride elimination?
β-hydrogen transfer
β-hydride elimination involves the formation of a ___ bond and an M-__ bond
β-hydride elimination involves the formation of a π bond and an M-H bond
What happens to metal alkyls that contain β-hydrogens?
Rapid elimination of the β hydrogens (provided a few conditions are met)
What conditions must be met for β-hydride elimination to occur?
- The complex must have an open coordination site and an accessible, empty orbital on the metal center
- The leaving group (-H) needs a place to land
- The M-Cα and Cβ-H must have the ability to align in a syn coplanar arrangement (all four atoms are in the same plane and the M-Cα and Cβ-H bonds are on the same side of the Cα-Cβ bond)
What is this reaction called? What will the unstable intermediate look like?
Beta-hydride reaction
How do you prevent molecules from undergoing beta-hydride elimination reactions?
- Coordinatively saturate the metal to tie up vacant orbitals
- Use ligands without beta-hydrogen atoms
True or False:
Both beta-hydrogen transfers and beta-hydride eliminations proceed through a 4-membered cyclic intermediate
True
What is the reverse reaction of oxidative addition?
Reductive elimination
What happens during reductive elimination?
The oxidation state of the metal decreases by 2 units and open coordination sites become available
What reaction types would be occurring if the oxidation state of the metal decreases by 2 units and open coordination sites become available?
Reductive elimination
Show how the following reaction is an example of reductive elimination
What is homolytic fission?
The cleavage of M-R bond to give two reactive radicals
What are the five methods of sigma complex synthesis?
Metallation
Redox Transmetallation
Metathesis
Salt formation
Oxidative addition
Predict the products of this reaction:
CH3Li + Ph3CH ->
What is the reaction type?
CH3Li + Ph3CH -> Ph3CLi + CH4
Metallation
Predict the products of this reaction:
2Al + 3Ph2Hg ->
What is the reaction type?
2Al + 3Ph2Hg -> 2AlPh3 + Hg
Redox Transmetallation
Predict the products of this reaction:
HgX2 + 2PhMgX ->
What is the reaction type?
HgX2 + 2PhMgX -> HgPh2 + MgX2
Metathesis
Predict the products of this reaction:
Na+[Mn(CO)5]- + CH3I ->
What is the reaction type?
Na+[Mn(CO)5]- + CH3I -> [CH3Mn(CO)5] + NaI
Salt formation
What conditions are needed for oxidative addition to proceed?
- Non-bonding electron density must be present on the metal
- Two vacant coordination sites normally exist on M
- Metal must have stable oxidation states separated by two units (sq planar -> octahedral)
What are the two product types of hydroformylation?
Linear and a branched product
