Fundamental organometallic reactions Flashcards
What are the basic types of organometallic reactions?
Basic types include ligand exchange, oxidative addition, reductive elimination, and olefin insertion.
These reactions are influenced by steric and electronic constraints, essential for understanding the reactivity and stability of organometallic complexes.
Describe the difference between dissociative and associative ligand exchange.
Dissociative exchange involves the loss of a ligand before another binds, favoured in complexes with high coordination numbers or are electronically saturated.
Associative exchange involves binding of an additional ligand before one detaches, favoured by low coordination numbers and electronic unsaturation.
What is the 16/18 electron rule and its importance in ligand exchange?
The 16/18 electron rule helps predict the stability of organometallic complexes: complexes tend to be stable when they have 18 electrons and less stable with 16. This rule is crucial in determining whether a ligand exchange is likely to be associative or dissociative.
Calculate the electron count for Rh(I) in a ligand substitution step.
In ligand substitution, Rh(I) initially has 16 electrons; after losing one ligand and gaining another, the electron count may vary but often returns to 16 or 18, ensuring stability according to the 16/18 electron rule.
What is olefin insertion and its key constraints?
Olefin insertion involves the integration of an olefin into a metal-hydride bond, leading to the formation of an alkyl metal complex.
Key constraints include the need for cis alignment of the olefin and hydride and the electron count decreasing by 2 so must start from 18e- complex
Describe β-Hydride Elimination and its relationship with olefin insertion.
β-Hydride Elimination is the reverse of olefin insertion; it involves the elimination of a β-hydride from alkyl ligand to form a hydride and olefin ligands.
Outline the process and constraints of alkyl migration/CO insertion.
In this reaction, an alkyl group migrates from the metal to a coordinated CO, forming an acyl complex.
- 2 ligands become 1
- ligands must be cis
- electron count decreases by 2 (oxi state not affected)
- start from 18e- complex
What is oxidative addition?
Metal donates 2 e- into X-Y bond:
- Oxidation state of metal is increased by 2 - must have stable oxi state 2 above initial
- electron count increases by 2 - gives 2 new ligands
- must have vacant coordination sites
- start from 16e- complex
Explain the different mechanisms of oxidative addition.
Mechanisms include concerted (ligands add simultaneously and cis), SN2 (nucleophilic attack on an electrophile with cis/trans products), and SN1 (dissociative with cis/trans products).
What is reductive elimination and how does it function?
Reductive elimination is the reverse of oxidative addition; it involves the formation of a bond between two ligands on a metal, resulting in their detachment and reduction of the metal’s oxidation state. It usually occurs from a complex saturated with ligands.
Determine the formal oxidation state, metal d-electron count, and complexes’ valence electron counts for [Ir(CO)2(CH3)I3], and [Pt(CH2=CH2)Cl3]-.
[Pt(CH2=CH2)Cl3]-: Oxidation state of Pt is +2, d-electron count is 8, total electron count is 16.
Explain the detailed mechanism of olefin insertion involving [RhH(CO)(PPh3)2].
Olefin insertion with Rh(I) starts with 18 electrons, inserting an olefin reduces ligand count by 1 and decreasing electron count by 2, becoming a 16e- complex.
Reaction proceeds by metalacyclic transition state, olefin and H- must be cis to occur
Detail the pathway of β-hydride elimination in metal complexes.
Starting from a complex with an metal alkyl bond, β-hydride elimination regenerates a hydride and an olefin, reversing the insertion process, influenced by metal’s coordination environment.
Forms cis hydride and olefin ligands - 2 ligands become 1
- electron count increases by 2
Discuss different types of oxidative addition mechanisms with visual aids.
Includes SN2-like attacks leading to cis or trans products, SN1-like pathways with possible cis or trans outcomes, and concerted pathways where X-Y bond addition impacts metal’s oxidation state (increases by 2) and coordination (addition of 2 ligands), forming cis product.
