Question

Which of the following ions is(are) expected to form colored octahedral aqueous complex ions? a. \(Z n^{2+}\) b. \(C u^{2+}\) c. \(M n^{3+}\) d. \(T i^{4+}\)

Short answer

The ions \(Cu^{2+}\) (b) and \(Mn^{3+}\) (c) are expected to form colored octahedral aqueous complex ions as they have partially filled d-orbitals.

Step-by-step solution

Identify the electronic configurations of the given ions

To determine whether a given ion has a partially filled d-orbital, we need to know its electronic configuration. For this, we will first find the atomic numbers and electronic configurations of the neutral atoms of the given elements and then remove appropriate electrons to obtain the configuration of the ions. a. \(Zn^{2+}\): Zn has an atomic number of 30. The electronic configuration of Zn is \([Ar]3d^{10}4s^2\). When Zn loses 2 electrons to form the \(Zn^{2+}\) ion, its configuration becomes \([Ar]3d^{10}\). b. \(Cu^{2+}\): Cu has an atomic number of 29. The electronic configuration of Cu is \([Ar]3d^{10}4s^1\). When Cu loses 2 electrons to form the \(Cu^{2+}\) ion, its configuration is \([Ar]3d^9\). c. \(Mn^{3+}\): Mn has an atomic number of 25. The electronic configuration of Mn is \([Ar]3d^5 4s^2\). When Mn loses 3 electrons to form the \(Mn^{3+}\) ion, its configuration becomes \([Ar]3d^4\). d. \(Ti^{4+}\): Ti has an atomic number of 22. The electronic configuration of Ti is \([Ar]3d^2 4s^2\). When Ti loses 4 electrons to form the \(Ti^{4+}\) ion, its configuration is \([Ar]3d^0\).

Check for partially filled d-orbitals

Now, we will check which of the ions have partially filled d-orbitals (i.e., having d-electrons but not completely filled, which is 10 electrons). a. \(Zn^{2+}\): The configuration of \(Zn^{2+}\) is \([Ar]3d^{10}\), which has no partially filled d-orbitals, so it is not expected to form colored octahedral aqueous complex ions. b. \(Cu^{2+}\): The configuration of \(Cu^{2+}\) is \([Ar]3d^9\), which has partially filled 3d-orbitals, so it is expected to form colored octahedral aqueous complex ions. c. \(Mn^{3+}\): The configuration of \(Mn^{3+}\) is \([Ar]3d^4\), which has partially filled 3d-orbitals, so it is expected to form colored octahedral aqueous complex ions. d. \(Ti^{4+}\): The configuration of \(Ti^{4+}\) is \([Ar]3d^0\), which has no partially filled d-orbitals, so it is not expected to form colored octahedral aqueous complex ions.

Determine the answer

Based on our analysis, the ions expected to form colored octahedral aqueous complex ions are: - \(Cu^{2+}\) (b) - \(Mn^{3+}\) (c) Thus, the correct answer is: b and c.

Key concepts

Electronic configuration

When discussing transition metal complexes, understanding the electronic configuration is crucial. This configuration describes the arrangement of electrons in an atom's orbitals, particularly focusing on the valence electrons, which determine an element's chemical properties. When a transition metal forms an ion, electrons are typically removed from the outermost s orbital before any d orbitals are affected. For example, when copper (Cu) forms a Cu²⁺ ion, it loses two electrons: one from the 4s and one from the 3d orbital, resulting in the electronic configuration

• **Cu**: \[ [Ar] \, 3d^{10} \, 4s^1 \]
• **Cu²⁺**: \[ [Ar] \, 3d^9 \]

Similarly:

• **Manganese (Mn)**: \[ [Ar] \, 3d^5 \, 4s^2 \] results in **Mn³⁺**: \[ [Ar] \, 3d^4 \]
• **Zinc (Zn)**: \[ [Ar] \, 3d^{10} \, 4s^2 \] results in **Zn²⁺**: \[ [Ar] \, 3d^{10} \]
• **Titanium (Ti)**: \[ [Ar] \, 3d^2 \, 4s^2 \] results in **Ti⁴⁺**: \[ [Ar] \, 3d^0 \]

It's this rearrangement of electrons and the resultant configuration that determines the presence of partially filled d-orbitals.

Partially filled d-orbitals

Why are partially filled d-orbitals significant? In transition metals, d-orbitals play a crucial role in determining the metal's properties, especially in complex formation. These d-orbitals can accommodate up to 10 electrons, and when they are partially filled, they provide unique characteristics.
For an ion to be involved in forming colored complex ions, its d-orbitals should not be entirely empty or filled. For instance, in Cu²⁺, the configuration is \[ [Ar] \, 3d^9 \] indicating a partially filled d-orbital with 9 electrons. Similarly, Mn³⁺ has a configuration of \[ [Ar] \, 3d^4 \], with 4 electrons in its d-orbitals.

• These partially filled orbitals allow for electronic transitions, which contribute to the complex being colored.
• An ion like Zn²⁺, with \[ [Ar] \, 3d^{10} \], do not have the same potential for these transitions, due to the complete filling of d-orbitals and, as a result, are often colorless.

In contrast, Ti⁴⁺ with \[ [Ar] \, 3d^0 \] has empty d-orbitals, lacking the capacity for such electronic transitions.

Colored complex ions

The hallmark of transition metal chemistry is the formation of colored complex ions, which arises from specific electronic processes. When a transition metal ion with partially filled d-orbitals forms a complex with ligands, the d-orbitals split into different energy levels due to the ligand's electric field.
This splitting allows for electronic transitions between these sub-levels when the ion absorbs visible light, making the complex appear colored.

• For example, in **Cu²⁺** complexes, the configuration \[ [Ar] \, 3d^9 \] allows for transitions between low-energy and high-energy d-orbitals, resulting in a characteristic blue color.
• Similarly, **Mn³⁺** complexes, with \[ [Ar] \, 3d^4 \], can exhibit colors resulting from such internal d-d transitions.

This principle doesn't apply to ions like Zn²⁺ or Ti⁴⁺, as they either have fully filled or completely empty d-orbitals, thereby typically not forming colored complexes. Understanding these concepts is crucial for explaining why certain transition metal complexes exhibit color while others remain colorless.