Thermodynamic vs Kinetic Stability

Thermodynamic vs Kinetic Stability in Coordination Compounds

In coordination chemistry, the stability of metal complexes is described in two distinct ways: thermodynamic stability and kinetic stability. These concepts are independent, meaning a complex can be thermodynamically stable but kinetically labile, or vice versa.

Thermodynamic Stability

Thermodynamic stability refers to the extent to which a complex forms or remains intact at equilibrium. It measures how favorable the complex is energetically compared to its dissociated components.

• Quantified by the formation constant (K_f) or stability constant (β).
• High K_f indicates strong metal-ligand bonds and a favorable equilibrium (large negative ΔG).
• Depends on factors like chelate effect, crystal field stabilization energy (CFSE), metal ion charge/size, and ligand basicity.

Example: Complexes with chelating ligands (e.g., ethylenediamine) are more thermodynamically stable than those with monodentate ligands (e.g., ammonia) due to the chelate effect.

Kinetic Stability

Kinetic stability refers to the rate at which a complex undergoes ligand substitution or decomposition. It is about the activation energy barrier for reactions.

• Labile complexes: Fast ligand exchange (substitution in <1 minute).
• Inert complexes: Slow ligand exchange (high activation energy barrier).
• Determined by electronic configuration (e.g., high CFSE often leads to inertness), geometry, and mechanism (associative or dissociative).

Factors Determining Kinetic Inertness (Octahedral Complexes)

d-Electron Configuration	CFSE Loss in Transition State	Kinetic Behavior	Examples
d3	High (loses large CFSE)	Inert	Cr3+
d6 low-spin	High	Inert	Co3+, Fe2+
d4 low-spin, d5 low-spin	Moderate to high	Usually inert	Mn3+, Fe3+
d8 (square planar)	Low barrier	Labile	Ni2+, Pd2+, Pt2+
d1, d2, d4–d7 high-spin	Low	Labile	Most first-row transition metals

Differences

Aspect	Thermodynamic Stability	Kinetic Stability
Definition	Equilibrium position (how much complex exists)	Reaction rate (how fast substitution occurs)
Measurement	Stability constant (Kf or β)	Rate of ligand substitution (labile vs inert)
Focus	Energy difference between reactants and products	Activation energy barrier
Independence	A complex can be thermodynamically stable but kinetically labile, or thermodynamically unstable but kinetically inert.

Examples

• Thermodynamically stable and kinetically labile: [Ni(CN)₄]^2- – High K_f but rapid substitution.
• Thermodynamically stable and kinetically inert: [Co(NH₃)₆]³⁺ – High stability constant and slow ligand exchange (low-spin d⁶ configuration with high CFSE).
• Kinetically inert (often Cr³⁺ or Co³⁺ complexes): [Cr(NH₃)₆]³⁺ – d³ configuration leads to high activation energy for substitution.

Factors Influencing Stability

• Irving-Williams order for thermodynamic stability: Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > Zn²⁺.
• High CFSE (e.g., d³, low-spin d⁶) often correlates with both high thermodynamic stability and kinetic inertness.

Important Examples for Exams

Complex	Geometry	d Configuration	Thermodynamic Stability	Kinetic Behavior	Reason
[Ni(CN)4]2–	Square planar	d8 (Ni2+)	Stable (high β)	Labile	Square planar complexes undergo fast associative substitution
[Cr(CN)6]3–	Octahedral	d3 (Cr3+)	Stable	Inert	High CFSE, large CFSE loss in TS
[Mn(CN)6]3–	Octahedral	d4 low-spin (Mn3+)	Stable	Inert	Strong field CN–, significant CFSE
[Co(NH3)6]3+	Octahedral	d6 low-spin	Stable	Inert	Classic inert complex
[CoF6]3–	Octahedral	d6 high-spin	Less stable	Labile	Weak field F–

Quick Revision Tips for Exams:
• Thermodynamic stability → high β or K_f
• Kinetic inertness → d³, low-spin d⁶, low-spin d⁴/d⁵ octahedral; square planar d⁸ usually labile
• Strong-field ligands (CN⁻, CO, NH₃) → both stable & often inert (except Ni²⁺ square planar)
• Weak-field ligands (F⁻, H₂O for 2+ ions) → labile