Question

Which is the most stable oxidation state of Co and why?

Hint:

For first-row transition metals, +2 is often most stable oxidation state.

Answer 1

Step 1: Understanding the Oxidation States of Cobalt (Co).
Cobalt (Co) is a transition metal that can exist in various oxidation states, ranging from +1 to +3 and even +4 in some compounds. The most common oxidation states of cobalt are +2 and +3, with +2 being the most stable.

Step 2: The Stability of the +2 Oxidation State.
The most stable oxidation state of cobalt is +2. This is due to the electronic configuration of the Co²⁺ ion, which has the electron configuration [Ar] 3d⁷. The stability of this state is enhanced because of the relatively low ionization energy required to remove two electrons from cobalt, and because the 3d⁷ configuration is comparatively stable.

Step 3: Comparison with the +3 Oxidation State.
The +3 oxidation state of cobalt (Co³⁺) is less stable than +2 because the ion has a higher positive charge and a smaller radius. This increases the ion's effective nuclear charge, making it more prone to attracting electrons from surrounding ligands, which destabilizes the ion. Additionally, the electron configuration of Co³⁺ is [Ar] 3d⁶, which is less stable than the 3d⁷ configuration of Co²⁺ due to the greater repulsion between electrons.

Step 4: Influence of Crystal Field Stabilization Energy.
In complexes, the stability of the oxidation states of cobalt is also influenced by the crystal field stabilization energy (CFSE). For Co²⁺ in an octahedral field, the CFSE is generally higher compared to Co³⁺, further favoring the +2 oxidation state in many compounds.

Step 5: Conclusion.
In conclusion, the most stable oxidation state of cobalt is +2. This is because the Co²⁺ ion has a stable 3d⁷ electron configuration, and the energy required to achieve the +3 state is higher, making it less stable. Additionally, the crystal field stabilization for Co²⁺ is more favorable in many complexes, reinforcing the stability of the +2 oxidation state.