Ligand Substitution Quiz on Square Planar Complexes

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.