Coordination Chemistry Chapter 7

Kinetic Barrier

• Lower kinetic barrier for subsequent reattachment in chelate ligands compared to monodentate ligands due to proximity to the metal center

Chelate Effect

• Dissociation of ethylenediamine ligand is slower than that of ammonia due to necessary bending and rotation to move the free amine away from the metal
• Chelate effect causes polydentate complexes to be thermodynamically more stable than their monodentate counterparts
• Substitution for a chelated ligand is generally a slower reaction than that for a similar monodentate ligand

Trans Effect

• In square-planar Pt(II) compounds, ligands trans to chloride are more easily replaced than those trans to ammonia
• Chloride has a stronger trans effect than ammonia
• The trans effect controls the substitution of ligands in Pt(II) compounds
• Ligands trans to certain other ligands are easily substituted, with the controlling ligands being CO, CN-, olefins, H-, PR3, NO2-, I-, SCN-, Br-, Cl-, NH3, py, OH-, and H2O

Lability of Cl-

• The lability of chloride controls the substitution of ligands in certain reactions

Contribution of Two Factors

• Thermodynamic effect (trans influence): weakening of the Pt-X bond
• Kinetic effect (trans effect): stabilization of the 5-coordinate transition state

Trans-Influence

• The strong Pt-T sigma bond prevents the trans leaving group Pt-X bond from being strong
• The weak Pt-X bond corresponds to a high energy ground state
• The Ea required to get X to leave is small

The Kinetic Effect (Trans Effect) Contribution

• π-acceptors remove e- density from Pt, making association with Y more likely
• This interaction from Pt-T lowers the energy of the 5-coordinate intermediate
• Ea is lowered and the Pt-X bond is more easily broken
• The metal center becomes more electrophilic for nucleophilic attack

Chapter 7: Thermodynamic and Non-Redox Kinetic Factors in Coordination Chemistry

• Objectives: mechanisms of substitution reactions of octahedral and square planar complexes

Hard-Soft Acids and Bases

• HSAB concept: Ag+ is soft and strongly bonds with Br- relative to F- and Cl-

Factors that Affect Stability of Complexes

• ∆G = -RTlnK = ∆H – T∆S allows calculation of free energy, entropy, and enthalpy of a reaction from stability constants

Reactions of Coordination Compounds

• Types of reactions: substitution, oxidation/reduction, ligand reactions, etc.
• Werner and Jorgenson discovered many of the basic reactions
• Experimentation over many years has yielded proposed mechanisms

Substitution Reactions

• Inert and labile complexes (kinetic stability)
• Labile complexes: those undergoing substitution with t½ < 1 minute
• Labile metal ions: those with small or zero LFSE
• Examples: d1, d2, d7, d9, d10 and high-spin d4-d6

Kinetic Stability versus Thermodynamic Stability

• Stability (formation) constant = 2.0 x 10³¹

Substitution Reactions

• These reactions can produce colored products used to identify metal ions
• Water exchange rate constants vary widely as a function of the metal ion
• Labile complexes: those with a very low activation energy for ligand substitution

Inert and Labile Complexes

• Inert complexes: those that react more slowly, with a higher activation energy for ligand substitution
• Werner studied Co(III), Cr(III), Pt(II), and Pt(IV) compounds because they are inert and more readily characterized than labile compounds

Inert Octahedral Complexes

• Inert octahedral complexes are generally those with high LFSE, specifically those with d3 or low-spin d4 through d6
• Examples: [Cr(H2O)6]3+ undergoes water exchange exceedingly slowly relative to the high-spin d4 [Cr(H2O)6]2+