Q6(a)(ii). Labile complexes; lability and d-electron configuration

1. Labile and inert: terms of kinetic stability. A complex that undergoes substitution quickly is labile; one that reacts slowly is inert. Thermodynamic stability is a different matter: [Co(NH₃)₆]³+ is thermodynamically unstable in acid yet inert, and keeps for years in solution.
2. Size and charge: small, highly charged ions hold their ligands longer. Water exchange is about 10⁵ s⁻¹ for Mg²+ and about 10⁸ s⁻¹ for the larger Ca²+.
3. Electronic structure (Taube): labile: all complexes with electrons in the antibonding e_g* orbitals (high-spin Co²+ t_(2g)⁵e_g², high-spin Fe³+ t_(2g)³e_g², Cu²+; Ni²+, d⁸, is labile but slower) and all with fewer than three d electrons (Ti³+, d¹). Inert: d³ (Cr³+, t_(2g)³) and low-spin d⁴, d⁵, d⁶ (Co³+, t_(2g)⁶).
4. Why: in a dissociative path the octahedron becomes a five-coordinate square pyramid. The change in CFSE is the ligand-field activation energy (LFAE, panel): it is large and positive for d³, d⁸ (+2.00 Dq) and low-spin d⁶ (+4.00 Dq), which raises the barrier (inert); zero or negative for d⁰–d², high-spin d⁴–d⁷, d⁹, d¹⁰ (labile).
5. The leaving group matters too: for [Co(NH₃)₅X]ⁿ⁺ + water, k=5.8×10⁻¹² s⁻¹ for X = NH₃ (t_(1/2) 3800 years) but 1.0×10⁻² s⁻¹ for X = ClO₄⁻ (t_(1/2) 7 s).

Methods: labile complexes react within seconds to nanoseconds, too fast for ordinary mixing. They are studied by fast-reaction techniques: stopped-flow (milliseconds), relaxation methods such as temperature jump and pressure jump (microseconds), and NMR line broadening, e.g. ¹⁷O NMR of water exchange. Inert complexes are followed by conventional methods and isotopic labelling (¹⁸O).

Application. inert Co(III) and Cr(III) complexes could be isolated as isomers and so built Werner's coordination theory; the inertness of Pt(II)–N bonds is why cisplatin survives long enough in the body to reach DNA.