Question

Why does anhydrous nickel chloride (NiCl2) appear yellow, even though chloride ions are considered weak field ligands, which should result in a smaller ligand field splitting energy and the absorption of longer-wavelength light, while nickel chloride hexahydrate (NiCl2·6H2O) appears green, despite the presence of water, a strong field ligand that should lead to a larger ligand field splitting energy and the absorption of shorter-wavelength light? Can someone clarify this apparent contradiction?

In this example, NiCl2 absorbs in the violet region of Vis-light (high energy) as a solid, so appears yellow, although chloride ions are weak field splitting ligands according to the spectroscopic series. NiCl2 hexahydrate appears green in solid form, so absorbs in the red region of Vis-light (low energy), although water is a comparatively strong field splitting ligand. Why?

Answer 1

The color of anhydrous nickel chloride is yellow because it absorbs higher-energy violet light, while nickel chloride hexahydrate appears green because it absorbs lower-energy red light.

Step-by-step explanation:

The color of coordination compounds is determined by the energy levels of the d orbitals of the central metal atom and the specific ligands coordinated to it. In the case of anhydrous nickel chloride (NiCl2), even though chloride ions are weak field ligands, the compound appears yellow because it absorbs higher-energy violet light.

On the other hand, nickel chloride hexahydrate (NiCl2·6H2O) appears green because it absorbs lower-energy red light, despite the presence of water, a strong field ligand.