Question

The pair of the compounds in which both the metals are in the highest possible oxidation state is (a) \(\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}, \mathrm{MnO}_{2}\) (b) \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}, \mathrm{MnO}_{4}^{-}\) (c) \(\mathrm{TiO}_{3}, \mathrm{MnO}_{2}\) (d) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-},\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}\)

Short answer

Option (b): \ \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\) and \ \(\mathrm{MnO}_{4}^{-}\).

Step-by-step solution

Identify the Highest Possible Oxidation State for Each Metal

In the periodic table, each metal has a maximum oxidation state equal to its group number. For example, manganese (Mn) is in Group 7, so its maximum oxidation state is +7. Chromium (Cr) is in Group 6, so its maximum oxidation state is +6. Cobalt (Co) is in Group 9, with a max oxidation state of +3. Iron (Fe) in Group 8 has a max of +3. Titanium (Ti) is in Group 4, so its maximum oxidation state is +4.

Determine Oxidation State in Each Compound - First Compound in Pair

Evaluate the oxidation state of the metal in the first compound of each pair. For option (a) \\(\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}\), the charge of the cyanide ligand (CN)^- is -1, and the overall complex has a charge of -3. Hence, Co must be in the +3 oxidation state. Similarly, in option (d) \\(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\), the charge of Fe is +3. For option (b) \\(\mathrm{CrO}_{2}\mathrm{Cl}_{2}\), Cr is in the +6 oxidation state (oxygen at -2 for each O, Chlorine usually assumed at -1 in compounds). In option (c) \\(\mathrm{TiO}_3\), each oxygen is -2, so Ti is +6, which exceeds its max +4, indicating corruption in this option.

Determine Oxidation State in Each Compound - Second Compound in Pair

Evaluate the oxidation state of the metal in the second compound of each pair. For option (a) \\(\mathrm{MnO}_{2}\), Mn is +4 (Oxygen at -2). In option (b) \\(\mathrm{MnO}_{4}^{-}\), Mn is +7 (Oxygen at -2 each, giving a total of -8 with a -1 charge on ion gives Mn +7). For option (c), \\(\mathrm{MnO}_{2}\) also has Mn at +4 state. In option (d), \\(\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}\), Co is +3.

Compare Oxidation States to Maximum Values

Compare the calculated oxidation states to their maximum values: In option (a), Co is at max +3 but Mn is only +4, not at max +7. In option (b), Cr is at its max +6 state and Mn is at his max +7 state. In option (c), neither matches their max state. In option (d), both metals don't match their respective max states.

Determine the Correct Pair

The only pair where both metals are in their highest possible oxidation state is option (b) \\(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\) (Cr at +6) and \\(\mathrm{MnO}_{4}^{-}\) (Mn at +7).

Key concepts

Periodic Table and Group Numbers

To understand the oxidation states of transition metals, it's helpful to begin with the periodic table and group numbers. Transition metals are found in groups 3 to 12 of the periodic table. The group number of a transition metal often corresponds to the maximum oxidation state the element can exhibit.
For example:

• Manganese (Mn) is located in group 7, so its maximum oxidation state is +7.
• Chromium (Cr) falls under group 6, thus its maximum oxidation state is +6.
• Cobalt (Co), part of group 9, exhibits a maximum oxidation state of +3.
• Iron (Fe), in group 8, reaches a maximum oxidation state of +6, but in compounds like \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) it often exists in lower states like +3.
• Titanium (Ti) sits in group 4, with a +4 as its highest oxidation state.

Using the periodic table and group numbers can give a clear understanding of the potential oxidation states of a metal, a vital step when evaluating the chemistry of transition metal compounds.

Evaluation of Oxidation States

Determining the oxidation state of a transition metal in a compound involves a few critical steps. Firstly, consider the charges on non-metal atoms or polyatomic ions that form part of the metal complex. For instance, oxygen typically carries a -2 charge, and in coordination compounds, ligands like \(\mathrm{CN}^-\) bear specific charges as well. These charges are essential for calculating the metal's oxidation state.
Let's break down the calculation:

• In \(\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}\), each \(\mathrm{CN}^-\) contributes a -1 charge. Given the overall charge of the complex is -3, cobalt's oxidation state resolves as +3 to balance the charge.
• For \(\mathrm{CrO}_{2}\mathrm{Cl}_{2}\), each oxygen contributes -2 (total -4), with chlorine usually considered neutral in such calculations. Thus, chromium's oxidation state is +6 to offset the total negative two charge from oxygens.

By evaluating these contributions, one can correctly determine the oxidation states of metals in complex structures, crucial for identifying their chemical behaviors.

Transition Metal Compounds

Transition metal compounds exhibit diverse properties due to their variable oxidation states and coordination chemistry. These compounds significantly rely on the metal's ability to form multiple stable oxidation states, resulting in various forms of bonding and color.
The interplay between the metal center and its surrounding ions or molecules (ligands) defines these compounds.
Consider manganese (Mn), which in compounds like \(\mathrm{MnO}_{4}^{-}\), achieves its maximum (+7) oxidation state and shows strong oxidative properties. This diversity in oxidation states arises from the partially filled d-orbitals characteristic to transition metals, which can readily gain or lose electrons.
Transition metals are essential in catalysis and materials science due to their versatile coordination geometries and reactivity. Their complex chemistry allows them to act as catalysts in industrial processes, enhance material properties, and model biological systems.

Coordination Chemistry

Coordination chemistry involves studying the complex structures formed when transition metals bind with ligands. A ligand is an ion or molecule that donates a pair of electrons to a metal atom to form a coordinate bond. This results in a coordination complex, or a complex ion.
Coordination number, the number of ligand bonds to the metal center, plays a pivotal role in the geometry and properties of a complex.These complexes are characterized by stability types and can have various shapes:

• The octahedral shape is common, such as in \([\mathrm{Co}(\mathrm{CN})_6]^{3-}\), where the cobalt metal center is surrounded by six cyanide ligands.
• Square planar and tetrahedral geometries also occur, each offering unique chemical properties.

Coordination complexes in transition metals are essential in biological functions, with hemoglobin being a prime example, and in industrial applications like catalysis.
The ability of transition metals to form strong and flexible bonds with different ligands underlines the significance of coordination chemistry in technological advancements.