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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 '19 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 '19 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 '19 at 19:42
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    $\begingroup$ en.wikipedia.org/wiki/Iron_pentacarbonyl $\endgroup$ – Curt F. Jul 18 '19 at 14:58
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Tetrakis(triphenylphosphano)palladium(0) or $\ce{[Pd(PPh3)4]}$ is a common catalyst in organic chemistry that features a palladium(0) core with four triphenylphosphane ligands. It is a bright yellow, sometimes described as canary yellow. See the image below (taken from Wikipedia, where a full list of authors is available); however, I feel the picture doesn’t quite do the actual compound justice.

Tetrakis(triphenylphosphano)palladium(0)

Colour is a feature of electronic transitions. Different orbitals have different energies. If the energy difference between an occupied and an unoccupied orbital happens to correspond to the energy of a photon ($E=h\nu=\frac{hc}{\lambda}$), the photon can be absorbed and the electron excited (subject to a couple of forbidden exchanges as usually defined in the Laporte rule and the spin rule). This absorption of photons is what we perceive as colour if $\pu{400 nm}<\lambda<\pu{700nm}$.

While energy levels of orbitals are affected by the overall charge of a complex, a correct energy difference can happen independent of partial or entire charge.

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