An aqueous solution of a transition metal ion reacts with potassium thiocyanate to produce a characteristic blood-red coloured solution. Which of the following is the formula of the coordination complex primarily responsible for this colour?

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The reaction between Fe ³⁺ ions and thiocyanate ions ( SCN ⁻ ) produces a characteristic blood-red coloured complex. Fe ³⁺( aq ) + SCN ⁻( aq ) [ Fe ( SCN )]²⁺( aq ) The blood-red colour is due to the formation of the [ Fe ( SCN )]²⁺ complex (often represented with water ligands as [ Fe ( H ₂ O )₅( SCN )]²⁺ ). [ Fe ( SCN )₆]³⁻ is a common misconception for the primary species formed. [ Fe ( CN )₆]³⁻ is the ferricyanide ion which forms Prussian blue with Fe ²⁺ , and [ Co ( SCN )₄]²⁻ forms a blue complex. Answer: [ Fe