I observed some trends as follows:-

  • $\ce{PbCl2}$ white and $\ce{PbI2}$ yellow
  • $\ce{SnCl2}$ white, $\ce{SnCl4}$ red and $\ce{SnI4}$ black
  • $\ce{AgCl}$ white, $\ce{AgI}$, $\ce{AgBr}$ and $\ce{Ag2CO3}$ yellow

In all these cases as the size of anion increases ( polarisability increases) or charge on cation increases (polarising power increases) colour exhibited becomes darker.. How does increasing covalent character affect this? Or is there any other reason?

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    $\begingroup$ In silver compounds colour is mostly due to charge transfer. $\endgroup$ – najayaz Jan 23 '16 at 12:17
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    $\begingroup$ tin(IV) chloride is a colorless liquid and tin(IV) iodide is orange solid. $\endgroup$ – Nilay Ghosh May 7 '16 at 3:48

In molecular orbital theory, as the covalent character increases the homo-lumo gap decreases, and the excitation happens more easily. Thus the colour intensity increases.


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