Lecture 14_2025

Page 1: Overview of Square Planar Complexes

• Tetrahedral vs Square Planar Geometry for d8 metal ions

• Mechanism of Ligand Substitution for Square Planar Pt(II) complexes

• The Trans-Effect: Phenomenon influencing substitution reactions

• Square Planar Pt(II) Anticancer Drugs: Application of these complexes

• Course Reference: Lecture 14 – Square Planar Complexes, CHEM 2P32 - Winter Term 2022 Principles of Inorganic Chemistry

Page 2: Electronic Effects and Geometry Preferences

• Crystal Field Stabilization Energy (CFSE): Square planar complexes have greater CFSE.

• Low Spin Preference: Provided that Δo > P; low spin square planar geometry preferred for strong field ligands.

  • Case Examples:

    • Ni2+ (3d) favors square planar with strong field ligands.

    • Pt2+ and Pd2+ always exhibit square planar due to higher Δo values.

• Comparison of Coordination Complexes:

  • NiCl4^2- (Tetrahedral) - 2 unpaired electrons

  • Ni(CN)4^2- (Square Planar) - 0 unpaired electrons

  • PtCl4^2- (Square Planar) - 0 unpaired electrons

• Key Transitions: Tetrahedral favored only when Δo is small (P > Δo).

• Reactivity list for square planar complexes: Co+, Ni2+, Cu3+, Rh+, Pt2+, Ag3+, Ir+, Pd2+, Au3+

Page 3: Kinetics of Substitution Reactions

• Ligand Substitution Reactions Overview:

  • For Octahedral: ML5X + Y → ML5Y + X (dissociative mechanism).

  • For Square Planar: ML3X + Y → ML3Y + X.

• Order of Reaction Kinetics: Second-order kinetics: Rate = k[ML3X][Y]

  • Depends on characteristics of leaving group (X) and entering group (Y).

  • Steric Hindrance: Evidence of associative mechanism; high steric crowding slows reaction.

Page 4: Reaction Mechanism of Substitution

• Substitution Steps:

  • Incoming ligand approaches axial vacant site → square pyramidal intermediate.

  • Intramolecular rearrangement via trigonal bipyramidal intermediate → square pyramidal with incoming ligand in the basal plane.

  • Final step: Leaving group departs from axial site.

• Stereochemistry Preservation: Retained during substitution process.

Page 5: Associative Substitution Mechanism

• Key Mechanism Points:

  • Rate Law: Rate = k[PtL3X][Y]

  • Nucleophile (Y) attacks from top or bottom; leaving group (X) exits from opposite site.

  • Retention of stereochemistry throughout substitution.

  • Intermediate Formation: Transition state includes trigonal bipyramidal geometry.

• Factors Affecting Reaction Rate:

  • Nucleophilicity of Y

  • Nature of Leaving Group X

  • Influence of other ligands and trans-directing effects

  • Metal center influence.

Page 6: Factors Affecting Substitution Rate

• Nucleophilicity Series: H2O < NH3 ~ py < Br < I < CN-

  • Soft nucleophiles lead to strong M-Y bonds -> fast kinetics.

  • Hard nucleophiles yield weak Pt–Y bonds -> slow kinetics.

• Bond Formation Details: RDS involves bond formation to nucleophile Y and bond weakening to leaving group X.

Page 7: Leaving Group Impact on Rate

• Nature of Leaving Group (X):

  • Order of leaving groups: H2O > Cl- > Br- > I- > N3- > SCN- > NO2- > CN-

  • Stronger M-X bonds result in slower reaction rates due to destabilized transition states.

  • Soft ligands create strong M-X bonds leading to high-energy transitions and slow kinetics.

Page 8: Summary of Reaction Rate Influences

• Reaction Rate Influences:

  • Destabilizing the Ground State: Weakening M-X bond accelerates reaction.

  • Stabilizing the Transition State: Strengthening M-Y bonds enhances reaction rate.

Page 9: Steric Effects on Reaction Rates

• Sterics Influence on Rate:

  • Steric groups typically slow associative reaction rates.

  • Examples of kinetics relevant to R and R' substitutions (e.g. R=H, R'=Me) reflect this trend.

  • Increasing bulk on pyridine ligand impacts substitution speed.

Page 10: Definition of the Trans Effect

• Trans Effect: The influence of one coordinated ligand (trans) on the substitution rate of the ligand opposite it.

• Trans Effect Series: CO ~ CN- > PR3 > H- > Me- > NO2- ~ I- > SCN- > Br- > Cl- > py > NH3 > F- ~ OH- > H2O

• Applications: Understanding geometric isomer formation based on reaction dynamics.

Page 11: Practical Implications of the Trans Effect

• Practical Use: Cl- shows a stronger trans directing effect compared to NH3. Geometric isomer outcomes depend on the order of reagent addition.

Page 12: Example of Trans Effect in Synthesis

• Synthesis Case: Preparation of cis- and trans-PtCl2I(py) from PtCl4^2-, I-, and py.

• Note: Chloride ligands are typically replaced more readily than other ligands in substitution reactions.

Page 13: Theoretical Foundations of the Trans Effect

• π Backbonding: Ligands with π or π* orbitals accept electron density, enhancing trans effect in ligand series.

• Polarizability: Polarization enhances trans directing ability; less polarizable ligands rank lower in the trans effect series.

Page 14: Polarization Effect Explained

• Polarization Effect: Larger and more polarizable ligands weaken M-X bonds in ground states (trans influence).

• Strong M-L bonds can weaken opposing M-X bonds, accelerating reaction rates.

Page 15: Supporting Theories for Trans Effect

• Reactivity and Polarizability: Ligands show greater trans effects corresponding with increased polarizability.

• Observation: Pt(II) complexes indicate stronger trans effects compared to less polarizable Pd(II) and Ni(II).

Page 16: π-Bonding Theory

• Transition State Stabilization: Ligands with vacant π orbitals strengthen the stability of trigonal bipyramidal transition states, decreasing the activation energy (Ea).

• Example: Substitution favoring ligands that stabilize the Pt-Cl bond reduces substitution barriers.

Page 17: Metal Center Effects on Substitution

• Metal Center Influence: Reactivity order: Ni(II) > Pd(II) >> Pt(II).

• Ease of 5-coordinate complex formation leads to higher transition state stabilization, enhancing bimolecular rate performance.

Page 18: Summary of Trans Effect Characteristics

• Trans Effect Optimization:

  • Strong σ-donor and π-acceptor ligands accelerate substitution reactions.

  • Understanding reaction profiles under kinetic control aids in predictive control over stereochemical outcomes.

Page 19: Square Planar Pt(II) Anticancer Drugs

• Anticancer Drug Overview: Cis-platin and analogs consist of a Pt(II) center with 2 Cl and 2 NH3 ligands in cis configuration.

• Historical Context: First synthesized by M. Peyrone in 1884; rediscovered by Rosenberg in the 1970s.

Page 20: DNA Binding Mechanism of Cis-Platin

• Cis-platin Functions: Induces interstrand and intrastrand cross-linking, inhibiting DNA unwinding and replication.

• Trans-Platin: Inactive due to inability to form critical DNA cross-links.

Page 21: Interaction with DNA

• Primary Target: Cis-platin primarily alters DNA physical structure to prevent replication, engaging with vital proteins for cell division.

• Therapeutic Applications of Cis-platin:

  • Testicular (most effective)

  • Ovarian

  • Head and Neck

  • Bladder

  • Cervical

  • Lymphomas