The complex anion [FeCl₄]⁻ is yellow whereas the species [Fe₂Cl₆] trapped in an argon matrix is reddish. Dissolution of 0.1 mol FeCl₃(s) in 1 dm³ of either POCl₃ or PO(OR)₃ produces a reddish solution that turns yellow on dilution. Titration of the red solution in POCl₃ with Et₄NCl solutions leads to a sharp colour change (from red to yellow) at a 1:1 mole ratio of FeCl₃/Et₄NCl. Vibrational spectra suggest that oxochloride solvents form adducts with typical Lewis acids by coordination of oxygen. Compare the following two sets of reactions as possible explanations of the observations.

Both equilibria are shifted to products by dilution.

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(a) Fe₂Cl + 2 POCl₂ ⇒ [FeCl₂] + 2 [POCL₂]+ POCI; + Et NC1 Et₂N+ + POCI₂ (b) Fe₂Cl +4POCl₂ ⇒ [FeCl₂(OPC13)4]++ [FeCl₂]