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?
