Q5(c)(ii). Co²⁺ salts stable, Co²⁺ complexes easily oxidised
Explain why— (ii) Co²⁺ salts are stabler than those of Co³⁺ salts, but solutions of Co²⁺ complexes get readily oxidized to Co³⁺ complexes. (At. No. of Co is 27)
Answer in brief
Co³⁺(aq) is so strong an oxidant (E°=+1.92 V) that it oxidises water, so simple salts are Co(II). Strong-field ligands stabilise Co(III), low-spin d⁶ (t2g⁶, CFSE -2.4 Δₒ), far more than Co(II); E° falls to about +0.1 V for [Co(NH₃)₆]³⁺/[Co(NH₃)₆]²⁺, and air oxidises the Co(II) complex.
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All parts of Question 5
- 5(a)(i) Fluorescence5 marks
- 5(a)(ii) Chemiluminescence5 marks
- 5(b)(i) Chelate effect5 marks
- 5(b)(ii) Lability5 marks
- 5(c)(i) Actinide spectra and the ligand5 marks
- 5(c)(ii) Co²⁺ salts stable, Co²⁺ complexes easily oxidised5 marks
- 5(d)(i) Shapes of the d orbitals5 marks
- 5(d)(ii) Splitting of d orbitals in a square-planar field5 marks