The colour of the transition metal ions is//are due to:

Options

A. d-d transition of electrons in presence of ligands
B. charge transfer from ligand to metal ion.
C. change in the geometry
D. Polarisation of anion by cation

Explanation

### Step-by-Step Text Solution:

1. **Understanding the Color of Transition Metal Ions**: The color of transition metal ions is primarily due to the presence of unpaired electrons in their d-orbitals. When these metal ions are in the presence of ligands, their d-orbitals split into two different energy levels.

2. **D-Orbital Splitting**: In the presence of ligands, the d-orbitals split into two sets: the lower energy set (often referred to as T2G) and the higher energy set (referred to as EG). This splitting occurs due to the electrostatic interactions between the ligands and the d-electrons.

3. **D-D Transition**: If there are unpaired electrons in the d-orbitals, they can absorb energy from visible light to get promoted from the lower energy level (T2G) to the higher energy level (EG). This process is known as a D-D transition. When the electron returns to its original state, it emits energy in the form of light, which we perceive as color.

4. **Example of Manganese Ion**: For example, consider the Mn²⁺ ion, which has an electronic configuration of [Ar] 3d⁵. When ligands approach, the 3d orbitals split, and the unpaired electrons can absorb visible light to undergo D-D transitions, resulting in the emission of color.

5. **Ligand to Metal Charge Transfer (LMCT)**: In cases where there are no unpaired electrons (for example, in certain complexes like permanganate or chromate), the color can still be observed due to ligand-to-metal charge transfer (LMCT). In this process, a non-bonding electron from the ligand can transfer to an empty d-orbital of the metal ion, leading to color.

6. **Conclusion**: Therefore, the color of transition metal ions can be attributed to two main phenomena:
- D-D transitions of electrons in the presence of ligands.
- Charge transfer from ligands to metal ions.

### Final Answer:
The color of transition metal ions is due to:
1. D-D transitions of electrons in the presence of ligands.
2. Charge transfer from ligand to metal ion.