Which one of the following complexes will most likely absorb visible light? (At. nos. Sc = 21, Ti = 22, V = 23, Zn = 30)

To determine which complex is most likely to absorb visible light, we need to understand that absorption of visible light in transition metal complexes occurs due to d-d transitions. These transitions happen when an electron moves from a lower energy d orbital to a higher energy d orbital within the same subshell. For this to occur, the central metal ion must have partially filled d orbitals. If the d orbitals are completely empty (d⁰) or completely filled (d¹⁰), no d-d transitions are possible, and the complex will not absorb visible light in the typical range for such transitions.

Let's examine the electron configurations of the metal ions in each complex:

• Scandium (Sc, atomic number 21): The neutral atom has configuration [Ar] 4s² 3d¹. For Sc³⁺, it loses three electrons (two from 4s and one from 3d), resulting in [Ar] 3d⁰.
• Titanium (Ti, atomic number 22): Neutral atom is [Ar] 4s² 3d². For Ti⁴⁺, it loses four electrons (two from 4s and two from 3d), giving [Ar] 3d⁰.
• Vanadium (V, atomic number 23): Neutral atom is [Ar] 4s² 3d³. For V³⁺, it loses three electrons (two from 4s and one from 3d), so the configuration is [Ar] 3d².
• Zinc (Zn, atomic number 30): Neutral atom is [Ar] 4s² 3d¹⁰. For Zn²⁺, it loses two electrons from the 4s orbital, resulting in [Ar] 3d¹⁰.

Now, analyze each complex:

Option A: [Sc(H₂O)₆]³⁺
The central ion is Sc³⁺, which has a 3d⁰ configuration. Since there are no d electrons, d-d transitions cannot occur. Water (H₂O) is a weak field ligand, but even with splitting, no electrons are present to transition. Thus, this complex does not absorb visible light via d-d transitions.

Option B: [Ti(NH₃)₆]⁴⁺
The central ion is Ti⁴⁺, with a 3d⁰ configuration. Again, no d electrons are present, so d-d transitions are impossible. Ammonia (NH₃) is a strong field ligand, but without electrons, splitting is irrelevant. This complex will not absorb visible light due to d-d transitions.

Option C: [V(NH₃)₆]³⁺
The central ion is V³⁺, with a 3d² configuration. Partially filled d orbitals allow d-d transitions. Ammonia (NH₃) is a strong field ligand, creating a large splitting energy (Δ₀) in the octahedral complex. For d², the electrons can occupy the t₂g orbitals, and transitions to the e_g level are possible. This complex can absorb visible light.

Option D: [Zn(NH₃)₆]²⁺
The central ion is Zn²⁺, with a 3d¹⁰ configuration. The d orbitals are fully filled, so no empty orbitals are available for d-d transitions. Ammonia (NH₃) causes splitting, but electrons cannot move within the d subshell as all orbitals are occupied. Thus, this complex does not absorb visible light via d-d transitions.

Comparing all options, only [V(NH₃)₆]³⁺ (option C) has a metal ion with partially filled d orbitals (d²), enabling d-d transitions and absorption of visible light.

Hence, the correct answer is Option C.