36 Questions
What happens to the oxidation state and coordination number of the metal in this process?
They remain unchanged
How many mechanisms are involved in this process?
Two
What is the rate-determining step in the dissociative mechanism?
The slow step
What type of liaisons are involved in the concerted mechanism with 3 centers?
Apolare or slightly polar liaisons
What is similar to the dissociative mechanism in organic chemistry?
SN1
What is the geometry of the product formed in the concerted mechanism?
Cis
What is the formula of the metal complex at the beginning of the process?
R3PNi(CO)3
What happens to the number of electrons during the reaction?
It decreases by two units
What is the product of the process?
Ni(CO)4
What type of addition is necessary for the resulting intermediate to stabilize?
Non-oxidative addition
How many types of mechanisms regulate the synthesis of Grignard reagents?
4
What happens to the coordination number of the metal in the dissociative mechanism?
It decreases
Which of the following can be coupled together?
All of the above
Which of the following mechanisms is analogous to the mechanism in organic chemistry?
SN2 mechanism
What is a characteristic of metals that makes oxydative coupling easier?
Basicity (rich in electrons)
What is the role of L in the dissociative mechanism?
It is a ligand
What is the general formula of the reactants in the synthesis of Grignard reagents?
R-X + Mg
What is the product of the synthesis of Grignard reagents?
RMgX
What is the general trend in the reactivity of alcynes versus alkenes?
Alcynes are more reactive than alkenes
What is the role of the metal complex in the reaction?
It acts as a catalyst
What is the result of the elimination of a ligand L in a complex?
The number of electrons in the complex decreases by 2 units.
What is the general purpose of the II-3 elimination reaction?
To permit the substitution of ligands L or to allow for oxidative addition.
What type of reaction is represented by the equation HCo(CO)4 → HCo(CO)3 + CO?
II-3 elimination reaction
What is the maximum number of electrons in the complex LnM in an oxidative addition reaction?
16
What is the oxidation state of the metal in the complex Ni(CO)3?
0
What type of ligands are involved in an oxidative addition reaction?
Ligands with 1 electron
What is the result of the reaction Pt(PPh3)3 + PPh3 → Pt(PPh3)4?
The addition of a ligand L
What is the general equation for the II-3 elimination reaction?
MXL4 → MXL3 + L
What is the advantage of using rhodium complexes over palladium complexes?
They offer better regioselectivity
What is the outcome of the elimination reaction (4) in the scheme?
Formation of an alkane
What is the role of the metal (Rh) in the scheme?
To catalyze the hydrogenation reaction
What is the reactant that undergoes hydrogenation in the scheme?
Alkene
What is the catalyst used in the hydrogenation reaction?
Rhodium
What is the net result of the reaction sequence?
Reduction of the alkene
What is the significance of the scheme?
It demonstrates the application of catalytic synthesis
Who is the author associated with the hydrogenation mechanism?
Halpern
Study Notes
Organometallic Reactions
- The degrees of oxidation and coordination of the metal, as well as the number of valence electrons, remain unchanged during the process.
- The process occurs according to two limiting mechanisms: dissociative and associative.
Dissociative Mechanism
- Resembles the SN1 mechanism in organic chemistry.
- Example: Ni(CO)4 → Ni(CO)3 + CO
- Rate-determining step: slow, followed by a rapid step.
Elimination Non-Reductive
- Generally, the non-oxidative addition (Lewis base) is equilibrated in solution with the reverse reaction, which involves the dissociation of the M-L bond, i.e., the elimination of a ligand L.
- Example: HCo(CO)4 → HCo(CO)3 + CO
- The reaction releases a coordination site, and the number of electrons in the complex decreases by 2 units.
Oxidative Addition
- Corresponds to the scheme: LnM + A → B → LnM
- Involves the addition of an electron-deficient ligand A, resulting in the formation of a 3-coordinate intermediate.
- Four types of mechanisms govern this type of reaction:
- Concerted 3-center mechanism
- SN2-like substitution mechanism
- Radical mechanism
- Ionic mechanism
Concerted 3-Center Mechanism
- Involves apolar or weakly polar bonds (H-H, O-O, H-Si, H-C, etc.)
- The product forms with a cis geometry, and the number of electrons decreases by 2 units, resulting in an unstable, non-isolable intermediate.
- A subsequent non-oxidative addition is necessary to stabilize the product.
Catalytic Applications
- Alkynes couple more easily than alkenes, and the coupling is facilitated by the basicity (electron richness) of the metal.
- It is possible to couple two carbene molecules or a carbene and an olefin.
- The alkane is obtained through reductive elimination (4) after insertion (3).
- The Halpern mechanism (1977) illustrates the catalytic cycle of olefin hydrogenation.
Catalysis with Palladium
- Rhodium complexes offer better regioselectivity than palladium complexes.
This quiz covers the process of metal complex formation and dissociation, including the dissociative and associative mechanisms. It involves the coordination and oxidation states of metal ions.
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