UO2n+ Properties: Analyzing Assertions

Let's analyze each assertion about the species UO\(_2\)\(^{n+}\) to determine which ones are correct.

UO2+ Stability in Water

Assertion A states that UO\(_2\)\(^{+}\) is thermodynamically stable in water. UO\(_2\)\(^{+}\) contains uranium in the +5 oxidation state (U\(^{V}\)). Uranium(V) species in aqueous solution are generally unstable with respect to disproportionation. Disproportionation is a reaction where a substance is simultaneously oxidized and reduced.

The typical disproportionation reaction for U(V) is:

\(2 \text{UO}_2\text{}^{+}_{(aq)} + 4 \text{H}^{+}_{(aq)} \rightarrow \text{U}^{4+}_{(aq)} + \text{UO}_2^{2+}_{(aq)} + 2 \text{H}_2\text{O}_{(l)}\)

In this reaction, U(V) is reduced to U(IV) (\(U^{4+}\)) and oxidized to U(VI) (UO\(_2\)\(^{2+}\)). Because this reaction readily occurs, UO\(_2\)\(^{+}\) is not thermodynamically stable in water. Therefore, assertion A is incorrect.

UO22+ Acid Character (Hard Acid)

Assertion B states that UO\(_2\)\(^{2+}\) is a hard acid. The concept of hard and soft acids and bases (HSAB principle) classifies chemical species based on their polarizability and charge density. Hard acids are typically small, highly charged cations that are not easily polarized. Soft acids are larger, have lower charges, and are more polarizable.

In UO\(_2\)\(^{2+}\), uranium is formally in the +6 oxidation state (U\(^{VI}\)). Although the charge is somewhat distributed in the polyatomic ion, the central uranium atom carries a significant effective positive charge. Uranium is a heavy element, but the high charge density around the uranium center makes it behave as a hard acid. Hard acids prefer to bond with hard bases (like oxide ions O\(^{2-}\) or water, which has oxygen as the coordinating atom), which aligns with the stability of the uranyl ion itself and its interactions with ligands in aqueous solutions. Therefore, assertion B is correct.

UO22+ Geometry

Assertion C states that the geometry of UO\(_2\)\(^{2+}\) is bent. The UO\(_2\)\(^{2+}\) ion, known as the uranyl ion, is well-established to have a linear structure. The two oxygen atoms are bonded directly to the uranium atom, forming a linear O=U=O arrangement. This linear geometry results from the involvement of uranium's atomic orbitals (5f, 6d, and 7s/7p) in bonding with the oxygen 2p orbitals, leading to strong covalent bonds and a stable linear structure. Therefore, assertion C is incorrect.

Uranyl Bonding Orbitals

Assertion D states that both the 5f and 6d orbitals of U are involved in bonding with the 2p orbitals of the oxygen atom in UO\(_2\)\(^{n+}\) species (referring to UO\(_2\)\(^{2+}\) in the context of the other assertions). Bonding in the uranyl ion is complex and involves significant covalent character. Molecular orbital descriptions of the uranyl ion show that uranium's 5f, 6d, and valence s and p orbitals combine with oxygen's 2p orbitals to form sigma and pi molecular orbitals. Specifically, both 5f and 6d orbitals of uranium participate in forming both sigma and pi bonds with the oxygen atoms. The strong covalent interaction involving these orbitals contributes to the stability and linearity of the uranyl ion. Therefore, assertion D is correct.

Identifying Correct Statements

Based on the analysis:

• Assertion A: UO\(_2\)\(^{+}\) is thermodynamically stable in water - Incorrect.
• Assertion B: UO\(_2\)\(^{2+}\) is a hard acid - Correct.
• Assertion C: The geometry of UO\(_2\)\(^{2+}\) is bent - Incorrect.
• Assertion D: Both the 5f and 6d orbitals of U are involved in bonding with the 2p orbitals of oxygen atom - Correct.

The correct statements are B and D.