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Questions and Answers
In a conjugate base mechanism, which ligand is more likely to be lost when deprotonation occurs?
In a conjugate base mechanism, which ligand is more likely to be lost when deprotonation occurs?
Which of the following ligands has the highest effect on reaction rate for substitution in square planar complexes?
Which of the following ligands has the highest effect on reaction rate for substitution in square planar complexes?
What does the trans effect describe in the context of square planar Pt(II) compounds?
What does the trans effect describe in the context of square planar Pt(II) compounds?
How do soft ligands compare to hard ligands when it comes to substitution reactions in metal complexes?
How do soft ligands compare to hard ligands when it comes to substitution reactions in metal complexes?
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What primary factor drives the loss of ligands in the trans effect?
What primary factor drives the loss of ligands in the trans effect?
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What type of compound is described as having a reaction half-life of 1 minute or less?
What type of compound is described as having a reaction half-life of 1 minute or less?
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Which substitution mechanism involves a low coordination number intermediate?
Which substitution mechanism involves a low coordination number intermediate?
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What do metal-ligand complexes typically undergo when reactions occur under conditions of excess incoming ligand?
What do metal-ligand complexes typically undergo when reactions occur under conditions of excess incoming ligand?
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Which of the following statements about labile and inert compounds is true?
Which of the following statements about labile and inert compounds is true?
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In a reaction following an associative mechanism, what typically happens during the first step?
In a reaction following an associative mechanism, what typically happens during the first step?
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Which of the following complexes is typically considered inert?
Which of the following complexes is typically considered inert?
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What characterizes the interchange mechanism in coordination reactions?
What characterizes the interchange mechanism in coordination reactions?
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Which mechanism is considered rare among coordination reactions due to the requirement for observable intermediates?
Which mechanism is considered rare among coordination reactions due to the requirement for observable intermediates?
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What is the order of the reactions when [Y] is large for both dissociation and interchange mechanisms?
What is the order of the reactions when [Y] is large for both dissociation and interchange mechanisms?
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In the association mechanism, what is the relationship between the rate law and the concentration of [Y]?
In the association mechanism, what is the relationship between the rate law and the concentration of [Y]?
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Which factor mentioned affects the rate of ligand exchange due to the oxidation state of the metal?
Which factor mentioned affects the rate of ligand exchange due to the oxidation state of the metal?
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What character do most octahedral reactions have according to the content provided?
What character do most octahedral reactions have according to the content provided?
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When both dissociation and interchange mechanisms have similar rate laws, what does it imply about the order of the reaction?
When both dissociation and interchange mechanisms have similar rate laws, what does it imply about the order of the reaction?
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What does the metal ionic radius influence in terms of ligand exchange rate?
What does the metal ionic radius influence in terms of ligand exchange rate?
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What type of mechanism characterizes the interaction when incoming ligands are close to the reacting species?
What type of mechanism characterizes the interaction when incoming ligands are close to the reacting species?
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What occurs in the second reaction of the association mechanism?
What occurs in the second reaction of the association mechanism?
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Study Notes
Coordination Chemistry IV: Reactions and Mechanisms
- Coordination compounds primarily undergo substitution reactions, many rapidly.
- Some substitution reactions are slower.
- Examples include:
- Cu(H₂O)₆²⁺ + 4 NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4 H₂O
- [Co(NH₃)₆]³⁺ + 6 H₃O⁺ → [Co(H₂O)₆]³⁺ + 6 NH₄⁺
Coordination Compound Reactions
- Labile compounds exhibit rapid ligand exchange (reaction half-life ≤ 1 min).
- Inert compounds exhibit slower reactions.
- Labile/inert classifications don't imply thermodynamic stability. Inert compounds can be thermodynamically unstable, but kinetics determine reaction rate.
- Inert compounds:
- Octahedral d³ complexes, low-spin d⁴-d⁶, strong field d⁸ square planar complexes
- Intermediate compounds: weak field d⁸ complexes
- Labile compounds: d¹, d², high-spin d⁴-d⁶, d⁷, dº, d¹º
Substitution Mechanisms
- Two main mechanisms, dissociative (D) and associative (A):
- Dissociative: low coordination number intermediate
- Associative: high coordination number intermediate
- SN1 or SN2 reactions are at the extreme limits of these systems.
- Interchange reactions involve incoming ligand participation without a detectable intermediate. Reactions can also exhibit characteristics of both associative and dissociative mechanisms.
- Reactions often proceed under conditions of excess incoming ligand.
- Mechanisms are primarily about octahedral complexes.
Classification of Substitution Mechanisms
- A table categorizes mechanisms based on the intimate mechanism (activation) and stoichiometric mechanism (transition state).
Determining Mechanisms
- Methods for determining reaction mechanisms are discussed.
Dissociation (D) Mechanism
- First step is ligand dissociation.
- A steady-state hypothesis assumes small [ML₅] and rapid intermediate consumption.
- Rate law suggests the intermediate must be observable.
- Dissociation mechanisms are rare; interchange-dissociative mechanisms are more common.
Interchange Mechanism
- First reaction is a rapid equilibrium where a ligand forms an ion pair or loosely bonded complex (not a high coordination number). The second step is slow.
- Rate laws show that both dissociation and interchange mechanisms have similar rate laws under different conditions of incoming ligand concentration.
Association (A) Mechanism
- First reaction results in an increased coordination number; the second reaction is faster. Rate laws are always second-order, regardless of incoming ligand concentration. Very few examples have a detectable intermediate.
Factors Affecting Rate
- Octahedral reactions mostly exhibit dissociative character with a possible square pyramid intermediate.
- High oxidation states lead to slower ligand exchange.
- Smaller ionic radii in metal ions show slower ligand exchange.
- Rates of transition metal reactions generally decrease down a group due to stronger M-L bonds.
Ligand Activation Energies
- A table presents activation energies determined by angular overlap, differentiating strong and weak field cases (octahedral and square-pyramidal transition states).
Evidence: Stabilization Energy and Rate of H₂O Exchange
- A graph displays stabilization energy and log of the half-life against the number of d electrons.
Entering Group Effects
- A table presents rate constants for anation or water exchange of [Co(NH₃)₅H₂O]³⁺ at 45 °C, with incoming ligands, such as H₂O, N₃⁻, SO₄²⁻, Cl⁻, and NCS⁻.
- Incoming ligand effect determines whether mechanism is dissociative or interchange.
Inner and Outer Sphere Electron Transfer Reactions
- Inner sphere mechanisms involve a direct interaction between reacting species, often with an intermediate.
- Outer sphere mechanisms don't involve a direct interaction and often have significantly different reaction rates despite the same basic mechanism.
- Outer sphere reaction tables provide oxidation and reduction agents and related rate constants.
Square Planar Reactions
- Substitution in square planar complexes frequently uses associative or dissociative mechanisms and square pyramid intermediates.
- Pt²⁺ is a soft acid.
- Ligand nature (hard/soft) can affect reaction rate.
- Leaving group (ligand loss) also affects rate.
Trans Effect
- In square planar Pt(II) compounds, trans ligands to Cl are more easily replaced than other ligands.
- CI has a strong trans effect.
- Ligands are more easily removed in certain geometries.
- Sigma bonds can be weaker where metal d orbitals have significant interaction.
- Pi bonding ligands weaken P-X bonds.
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Description
Test your knowledge about ligand substitution mechanisms in square planar complexes! This quiz covers crucial concepts like the conjugate base mechanism, the trans effect, and the comparison between soft and hard ligands. Assess your understanding of how these factors influence reaction rates and ligand loss in metal complexes.