42 Questions
What is the main difference between the Dewar–Chatt (D-C) model and the metalacyclopropane (MCP) model?
The main difference is the strength of back bonding, with the D-C model applicable for weak back bonding and the MCP model for strong back bonding.
Why does the alkene C=C bond length, dCC, increase on binding to the metal?
The alkene C=C bond length, dCC, increases on binding due to the depletion of the C=C π bond by donation to M and back donation from the metal that lowers the alkene C−C bond order.
What is the effect of electron-withdrawing substituents on the M-(alkene) bond?
Electron-withdrawing substituents on carbon encourage back donation and strengthen the M-(alkene) bond.
What is the difference in the C=C bond length between free C2H4 and Pt(II) complex?
The C=C bond length, dCC, is 1.337 Å in free C2H4 and 1.375 Å in the Pt(II) complex.
What is the extreme of the MCP model, and how does it resemble?
The extreme of the MCP model resembles the [C2H4]2– dianion with the carbons rehybridized from sp2 toward sp3.
How does the oxidation state of the metal affect the model that applies?
For Zeise’s salt and other intermediate oxidation state late metals, the D-C model fits best, while for Pt(0), the MCP model applies.
What is the difference in the C–H bonds between the D-C and MCP models?
In the MCP model, the C–H bonds fold back strongly, whereas in the D-C model, they do not.
What is the name of the model that applies to Pt(0) complexes, and why is it named so?
The model that applies to Pt(0) complexes is the metalacyclopropane (MCP) model, named so because it resembles 5.4, with LnM replacing one CH2 in cyclopropane.
What is the result of the p orbital on the carbon pointing more directly towards the metal in an η3-allyl complex?
The result is further improvement of the overlap.
What kind of exchange is often seen in the η3-allyl group?
Exchange of the syn and anti substituents.
What is the proposed mechanism of the Ru case in Eq. 5.28 and Eq. 5.29?
Oxidative addition of the dihalide to Ru(CO)3, formed by photolysis.
What is the common mechanism that involves an η1-allyl intermediate?
The mechanism involves an η1-allyl intermediate, as shown in Eq. 5.13.
What is the general reaction illustrated in Eq. 5.29?
Oxidative coupling of alkynes to give a metalacycle, followed by reductive elimination.
What is the significance of the nomenclature of syn and anti substituents?
The nomenclature is with respect to the central C–H.
How can the ligand 5.19 be best pictured on binding?
As an LX2 enediyl (5.20).
What is the first route to allyl complexes, as shown in Eq. 5.14?
The first route is from an alkene.
What is the result of the electrophilic attack on a diene complex in Eq. 5.17?
The result is an η3-allyl complex.
What is the result of the umbrella distortion from the ideal planar conformation of the ligand?
The central carbon is moved away from the metal, improving M–L overlap.
What is the significance of the cyclopentadienyl group (Cp) in organometallic transition metal π complexes?
It is the most firmly bound polyenyl and the most inert to both nucleophiles and electrophiles.
What is Markovnikov's rule, as shown in Eq. 5.18?
The hydrogen tends to add to the terminal carbon when a C=C group of a diene undergoes insertion into an M–H bond.
What is the result of the hydride adding to the central carbon in Eq. 5.19?
The result is an allyl complex.
What is the structure of ferrocene, Cp2Fe, discovered by Wilkinson, Woodward, and Fischer?
A sandwich structure.
What are the common types of complexes formed by the cyclopentadienyl group (Cp)?
Cp2M (metallocene) and CpMLn complexes (two-, three-, or four-legged piano stools where n = 2–4).
What is the limitation of the cyclopentadienyl group (Cp) as a spectator ligand?
It is not inert to strong oxidants.
What is the oxidation state of the metal M in the electron-deficient 16e species Cp2MX2?
M(IV)
How many electrons do the Cp2Mo and Cp2W fragments have before bonding with ligands?
16e
What happens to the nonbonding orbital in the metallocene dichloride Cp2MoCl2?
It becomes full and capable of back donation.
What type of ligands can stabilize the unsaturated ligands in the [Cp2M(C2H4)Me]+ complex?
Unsaturated ligands
What is the result of protonating the lone pair between the hydrides in Cp2MH2?
The formation of the water-soluble cations, [Cp2MH3]+.
How many available orbitals are used in the Cp2MH2 complex?
Two
What is the electron count of the Cp2Re fragment?
17e
How many X ligands can bind to the Cp2M fragments from the group 5 metals?
Three
What is the role of Tp ligands with substituents at the 5-position in some metal complexes?
They act as tetrahedral enforcers, allowing only a single additional ligand to bind.
What is the significance of [Cr(η6-C6H6)2] in the field of organometallic chemistry?
It was the first 'sandwich' compound to be identified, and its structure was proposed by Fischer and Hafner in 1955.
What is the typical mode of binding of arenes to metals?
The η6-form, where the arene binds through all six carbon atoms.
How does the binding of an arene to a metal affect the C-C distances within the ring?
The C-C distances become slightly longer than in the free arene.
What is the difference between arene and Cp ligands in terms of reactivity?
Arenes are more reactive than Cp groups, and are more easily lost from the metal.
What is the typical synthetic route for preparing arene complexes?
Reaction of the arene with a complex of a reduced metal, or reaction of the arene with a metal salt and a reducing agent.
What is the effect of arene binding on the electron density of the ring?
The electron density on the ring is depleted, making it susceptible to nucleophilic attack.
What is the role of reducing agents in the synthesis of arene complexes?
They facilitate the reduction of the metal, allowing it to bind to the arene.
What is the geometry of the η4 arene ring in metal complexes?
The η4 arene ring is strongly folded.
How do arene ligands differ from Cp ligands in terms of their behavior in metal complexes?
Arenes are more likely to act as actor ligands, whereas Cp ligands are more likely to act as spectator ligands.
Study Notes
Models of Metal-Alkene Binding
- The Dewar-Chatt (D-C) model holds for weak back bonding, while the metalacyclopropane (MCP) model applies for strong back bonding.
- Experimental structures can exhibit characteristics of both models, with the D-C model fitting best for Zeise's salt and other intermediate oxidation state late metals, and the MCP model applying for Pt(0).
- Both models are considered η2 structures.
Factors Affecting Alkene C=C Bond Length
- The M-alkene σ bond depletes the C=C π bond, slightly weakening and lengthening the bond.
- Back donation from the metal reduces the alkene C-C bond order, further increasing the bond length.
- The extent of back donation depends on the metal, with weakly π-basic Pt(II) resulting in a slight increase in bond length, and strongly π-basic Pt(0) resulting in a more significant increase.
Metalacyclopropane (MCP) Model
- The MCP model applies for strong back bonding, with the alkene C=C bond lengthening to 1.43 Å.
- The C-H bonds fold back strongly, with the carbons rehybridized from sp2 to sp3.
- The MCP extreme resembles a [C2H4]2- dianion, with LnM replacing one CH2 in cyclopropane.
Effects of Substituents on Alkene Binding
- Electron-withdrawing substituents on carbon encourage back donation and strengthen the M-alkene bond.
- For example, Pt(PPh3)2(C2CN4) has a dCC of 1.49 Å, approaching the C-C single bond length.
Synthetic Routes
- Synthetic routes for metal-alkene complexes include reactions involving oxidative addition, oxidative coupling, and reductive elimination.
Trimethylenemethane Complexes
- Trimethylenemethane is a highly unstable ligand in the free state, but can be stabilized by binding to a metal.
- The ligand exhibits an umbrella distortion, moving the central carbon away from the metal and improving M-L overlap.
Cyclopentadienyl Complexes
- Cyclopentadienyl (Cp) is a central ligand in organometallic transition metal π complexes, being the most firmly bound polyenyl and the most inert to nucleophiles and electrophiles.
- Cp is a reliable spectator ligand in metallocene and CpMLn complexes.
Synthesis of Cyclopentadienyl Complexes
- Synthetic routes for cyclopentadienyl complexes include reactions involving oxidative addition, oxidative coupling, and reductive elimination.
π-Complexes
- π-Complexes exhibit exchange of syn and anti substituents, which can affect the appearance of the 1H NMR spectrum.
- A common mechanism for this exchange involves an η1-allyl intermediate.
Arene Complexes
- Arene complexes are typically synthesized through reactions involving the arene and a reduced metal, or the arene, a metal salt, and a reducing agent.
- Arenes usually bind in the 6e, η6-form, but η4 and η2 structures are also seen.
- The C-C distances in arene complexes are usually essentially equal, but slightly longer than in the free arene.
Reactivity of Arene Complexes
- Arenes are more reactive than Cp groups and are more easily lost from the metal.
- Arenes are often actor ligands, rather than spectator ligands.
Test your understanding of the Dewar-Chatt and Metalacyclopropane models in organometallic chemistry, including their application to weak and strong back bonding, and the effects on alkene C=C bond length. Explore the differences between Zeise's salt and Pt(0) complexes.
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