CFSE of octahedral complexes is inversely related to the wavelength. Also, more the ligand is a strong field ligand, more will be the value of CFSE and lower will be its wavelength. Among the given complexes the strength of ligands are Br−(weaker), H2O and en(stronger). Since the emitted colours are red, green and blue, the absorbed colours are correspondingly green,red,orange. Hence the compounds are respectively blue red and green.

Understanding the Color of Complexes
The color of metal complexes is determined by their electronic configurations and the nature of ligands surrounding the metal ion. The splitting of d-orbitals in the presence of ligands leads to the absorption of specific wavelengths of light, resulting in the observed color.
Complexes and Their Colors
- [M(H2O)6]2+
- This complex typically forms with transition metals in an octahedral geometry. Water is a weak field ligand, leading to a smaller splitting of d-orbitals.
- The complex absorbs light in the red region, reflecting blue light, thus appearing blue.
- [M(en)3]2+
- Ethylenediamine (en) is a stronger field ligand than water. It creates a larger splitting of d-orbitals due to its bidentate nature.
- This larger splitting results in the absorption of light in the green region, causing the complex to appear red.
- [MBr6]4-
- Bromide ions are considered weak field ligands. In this complex, the larger number of bromide ligands leads to significant splitting.
- It absorbs light in the blue region, thus reflecting green light, making the complex green.
Summary of Colors
- [M(H2O)6]2+ → Blue
- [M(en)3]2+ → Red
- [MBr6]4- → Green
Conclusion
The color of these complexes is a direct consequence of the ligand field theory, where the type of ligands and their arrangement around the metal ion determine the electronic transitions and, ultimately, the color perceived by the observer. Understanding these relationships is crucial for the study of coordination chemistry in the context of NEET and other competitive examinations.