Question

______ ion forms colored aqua complexes.

• A. \(Cu^{2+}\)
• B. \(Zn^{2+}\)
• C. \(Ti^{4+}\)
• D. \(Mn^{7+}\)

Hint:

Colored transition metal complexes usually require partially filled \(d\)-orbitals.

Answer 1

The Correct Option is A

Concept:
Transition metal ions often form colored complexes because they have partially filled \(d\)-orbitals. The color usually arises due to: \[ d-d\ \text{transition} \] When ligands such as water approach the metal ion, the \(d\)-orbitals split into different energy levels. Electrons absorb visible light and jump between these split \(d\)-levels.

Step 1: Check \(Cu^{2+}\).
Copper has atomic number: \[ 29 \] Configuration of \(Cu^{2+}\): \[ Cu^{2+}=3d^9 \] Since it has partially filled \(d\)-orbitals, it can show \(d-d\) transition. The aqua complex of copper(II) is blue: \[ [Cu(H_2O)_6]^{2+} \]

Step 2: Check \(Zn^{2+}\).
\[ Zn^{2+}=3d^{10} \] It has completely filled \(d\)-orbitals. So it generally forms colorless complexes.

Step 3: Check \(Ti^{4+}\).
\[ Ti^{4+}=3d^0 \] It has no \(d\)-electrons. So \(d-d\) transition is not possible.

Step 4: Choose the correct ion.
Among the given ions, \(Cu^{2+}\) is the common ion that forms colored aqua complexes. Hence, the correct answer is: \[ \boxed{(A)\ Cu^{2+}} \]