Separate solutions of Co²⁺ and Ni²⁺ sulphates are each treated with excess KCN. On boiling the aerated-free solution, the cobalt solution slowly evolves a gas because its first cyanide complex reduces

Question

Separate solutions of Co²⁺ and Ni²⁺ sulphates are each treated with excess KCN. On boiling the aerated-free solution, the cobalt solution slowly evolves a gas because its first cyanide complex reduces water, whereas the nickel solution evolves no gas on boiling. Which statement is correct? Use spin-only reasoning and strong-field cyanide.

TestHub · Accepted Answer

Co²⁺ in excess cyanide gives [Co(CN)₆]⁴⁻, a low-spin d⁷ ion with one unpaired electron (so D is false). It reduces water to H₂ (one σ bond, not cyanogen, so C is false) and becomes [Co(CN)₆]³⁻, low-spin d⁶ and diamagnetic. Ni²⁺ gives [Ni(CN)₄]²⁻, square planar and diamagnetic, not tetrahedral paramagnetic (so B is false).

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