Why is the highest oxidation state of a metal exhibited in its oxide or fluoride only?
The highest oxidation state of a metal is exhibited in its oxide or fluoride due to the small size and high electronegativity of oxygen and fluorine. These properties enable them to oxidise the metal to its highest oxidation state.
For fluorine, the ability to stabilise the highest oxidation state is attributed to either higher lattice energy in ionic compounds like CoF₃, or higher bond enthalpy terms in covalent compounds like VF₅ and CrF₆.
Additionally, oxygen has the unique ability to form multiple bonds with metals, which explains why oxygen can stabilise even higher oxidation states than fluorine in some cases. For example, the highest Mn fluoride is MnF₄ whereas the highest oxide is Mn₂O₇.
Explanation
The textbook explicitly states in section 4.6 that small size and high electronegativity of oxygen and fluorine allow them to oxidise metals to their highest oxidation states. Section 4.3.7 elaborates that fluorine stabilises high oxidation states through higher lattice energy (ionic compounds) or higher bond enthalpy (covalent compounds). The text also notes oxygen's superiority due to its ability to form multiple bonds with metals, demonstrated by Mn₂O₇ (oxidation state +7) being higher than MnF₄ (oxidation state +4).