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?

  • $\begingroup$ Maybe the chloride ions are not acting as ligands in anhydrous nickel chloride, like in $\ce{NaCl}$ $\endgroup$
    – Maurice
    Oct 14 at 8:07


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