A complex ion, [M(OH)6]4-, is orange in solution. Which of the following ligands, when bound to the same metal might cause the solution to be blue?
Cl-
CN-
CO
F-
H2O
I-
NH3
NO2-
OH-
NCS-
To be orange the compound must absorb in the blue. To be blue the compound must absorb in the orange. Therefore, the first compound colored orange must be absorbing higher energy (blue is higher than orange). To become a blue compound it must have a lower energy; therefore, pick those ligands that have lower splitting levels than OH ligand has. Those should be listed in your text or notes. My listing shows this.
I^- < Br^- < Cl^- < F^- < OH^- < H2O < (COO^-)2 < NH3 <en < NO2^- < CN^-
To determine which ligand might cause the solution to be blue when bound to the same metal, we need to consider the color of the complex ion formed with each ligand.
The color of a complex ion depends on the electronic transitions that occur within the metal-ligand complex. The energy difference between the d-orbitals of the metal and the ligand field determines the color of the complex.
In general, ligands that are strong-field ligands cause a larger splitting of the d-orbitals and result in smaller energy differences between the split orbitals. This leads to absorption of higher energy light in the visible region, resulting in the complex appearing colored.
On the other hand, ligands that are weak-field ligands cause a smaller splitting of the d-orbitals and result in larger energy differences between the split orbitals. This leads to absorption of lower energy light (or no absorption) in the visible region, resulting in the complex appearing colorless or white.
Based on the spectrochemical series, which ranks ligands in order of increasing field strength, we can determine which ligands might cause the solution to be blue:
- Strong-field ligands that are likely to cause the complex to be blue: CN-, NO2-, NCS-
- Weak-field ligands that are less likely to cause the complex to be blue: Cl-, CO, F-, H2O, I-, NH3, OH-
Therefore, the ligands that might cause the solution to be blue when bound to the same metal are CN-, NO2-, and NCS-.