JEE Main 2017 — D And F Block Elements Question with Solution

In order to determine which pair of compounds have metals in their highest oxidation states, we need to consider the common oxidation states of the metals involved, as well as the rules for determining oxidation states in compounds and complex ions. Let's evaluate each option:

Option A: MnO₂ and CrO₂Cl₂

In MnO₂, manganese (Mn) has an oxidation state of +4. This is not the highest oxidation state manganese can achieve; it can go up to +7, as seen in KMnO₄ (potassium permanganate).

In CrO₂Cl₂, chromium (Cr) has an oxidation state of +6, which is indeed its highest oxidation state.

Thus, this option contains one metal in its highest oxidation state (Cr) but not the other (Mn).

Option B: [NiCl₄]^2- and [CoCl₄]^2-

For both [NiCl₄]^2- and [CoCl₄]^2-, the nickel (Ni) and cobalt (Co) are in a +2 oxidation state. Neither of these represents the highest oxidation state for these metals. Nickel can have a +3 state, and cobalt can go up to +4 in very rare cases.

Option C: [Fe(CN)₆]^3- and [Cu(CN)₄]^2-

In [Fe(CN)₆]^3-, the iron (Fe) has an oxidation state of +3, which is a common high oxidation state for iron, though iron can also exist in a +2 state commonly and +6 in very rare conditions. Therefore, +3 is considered a high oxidation state but not the absolute highest.

In [Cu(CN)₄]^2-, copper (Cu) has an oxidation state of +2, which is the highest stable oxidation state for copper in most of its compounds.

Option D: [FeCl₄]^- and Co₂O₃

In [FeCl₄]^-, iron (Fe) has an oxidation state of +3. As mentioned, +3 is a high state for iron, but not the absolute highest oxidation state it can achieve.

In Co₂O₃, cobalt (Co) has an oxidation state of +3, as evidenced by the formula, where two Co atoms interact with three O atoms (each oxygen providing a -2 charge, for a total of -6, requiring each Co to be +3 to balance the charge). +3 is indeed among the highest common oxidation states for cobalt, although, as mentioned, Co can technically reach +4.

Conclusion: Based on above analysis, while none of the options perfectly fits the criteria of both elements being in their absolute highest oxidation states (especially considering the rare or less common states), Option A and Option D come closest, with chromium and cobalt being in their common high oxidation states in the provided compounds. However, the question asks for the highest oxidation state, and strictly speaking, none of the options perfectly satisfy the condition for both elements included. Among the choices, Option A is the closest because chromium is in its highest oxidation state of +6 in CrO₂Cl₂, and manganese in MnO₂ is in a +4 state, which while not its absolute highest, represents a commonly encountered high oxidation state. Given the context and conventional exams' focus on common oxidation states, Option A would be the best fit despite the slight discrepancy with the question's phrasing.