Question

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-

Answer 1

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^-