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The iron complex $\ce{[Fe(H2O)6]^2+}$ is colored but I'm wondering, if a pair of chloride ion is inserted on its secondary valency to form [FeCl2(H2O)6], does the complex loose its color or not?

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    $\begingroup$ Outer sphere ions should not have a large impact on the color. $\endgroup$ – Zhe Jul 8 at 17:56
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    $\begingroup$ It seems the "if a chloride molecule is inserted on its primary valency" part might need clarification from your side. I suspect what you mean is a pair of chloride anions (not a molecule) replacing two water molecules within the main coordination sphere $(\ce{[Fe(H2O)4Cl2]}).$ Or are you asking about the color of $\ce{[Fe(H2O)6]Cl2}$ complex where chlorides are counterions? $\endgroup$ – andselisk Jul 8 at 18:13
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    $\begingroup$ The 'colour' arises from electronic transitions between the d to d energy levels ($t_{2g}$ and $e_g$) in the molecule. These energy levels are present whatever the ligand are, but different ligands alter their energy and so the colour observed. Sometimes this might lead to transitions in the uv or infra red which are outside our ability to see with our eyes, but we can still measure the spectrum using spectrophotometers. $\endgroup$ – porphyrin Jul 8 at 19:42
  • $\begingroup$ en.wikipedia.org/wiki/Iron_pentacarbonyl $\endgroup$ – Curt F. Jul 18 at 14:58

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