Stability of Coordination Compounds
1. Thermodynamic Stability
Refers to the Equilibrium state. How much complex forms?
- Metric: Stability Constant ($\beta$ or $K_f$).
- Driven by: $\Delta G$ (Energy difference between R and P).
- Trend: Higher $\beta$ = More stable.
2. Kinetic Stability
Refers to the Reaction Rate. How fast is the exchange?
- Metric: Rate Constant ($k$) / Activation Energy ($E_a$).
- Terms: Labile (Fast) vs. Inert (Slow).
- Driven by: CFSE loss in the Transition State.
Comparison & Coordination Trends
| Configuration | Kinetic Nature | Reasoning |
|---|---|---|
| $d^3$ (e.g., $Cr^{3+}$) | Inert | High CFSE; high barrier to change geometry. |
| $d^6$ Low-Spin (e.g., $Co^{3+}$) | Inert | Maximized CFSE; extremely stable electronic shell. |
| $d^8$ Square Planar ($Ni^{2+}, Pt^{2+}$) | Labile | Open coordination sites allow fast associative paths. |
| $d^0, d^{10}, d^5$ High-Spin | Labile | Zero or very low CFSE; no barrier to ligand loss. |