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Question:

Which of these statements about [Co(CN)₆]³⁻ is true?

[Co(CN)₆]³⁻ has four unpaired electrons and will be in a high-spin configuration.

[Co(CN)₆]³⁻ has no unpaired electrons and will be in a low-spin configuration.

[Co(CN)₆]³⁻ has no unpaired electrons and will be in a high-spin configuration.

[Co(CN)₆]³⁻ has four unpaired electrons and will be in a low-spin configuration.

Solution:

The electronic configuration of Co is [Ar]3d⁷4s². The central Co atom has +3 oxidation state and 3d⁶ as outer electronic configuration. Cyanide ion is a strong field ligand and Δo > P. Hence, pairing of electrons is preferred. Hence, the outer electronic configuration of Co³⁺ can be written as t₂g⁶eg⁰. Thus, [Co(CN)₆]³⁻ has no unpaired electrons and will be in a low-spin configuration.