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Looking at the electronic configurations of ferrous and ferric ions they are d6 and d5 systems respectively. Hence, ferric ions are more stable. But in octahedral complexes like hexathiocyanates, the electron count of ferric ions reaches only 17 while for ferrous it reaches 18. In the case of thiocyanate , ferric complexes with the ligands while ferrous does not react at all.

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  • $\begingroup$ And how do you measure "frequency"? $\endgroup$ – Mithoron Aug 22 '16 at 21:16
  • $\begingroup$ @Mithoron Can you be more clear? $\endgroup$ – Swaroop Chandra Aug 23 '16 at 1:31
  • $\begingroup$ That's exactly what I'm asking from you... Why do you say that one type of complexes would be more common/frequent or whatever? $\endgroup$ – Mithoron Aug 23 '16 at 12:58
  • $\begingroup$ @Mithoron my mistake.. I have edited the last part. $\endgroup$ – Swaroop Chandra Aug 26 '16 at 3:10
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    $\begingroup$ Why should 18e rule apply to typical coordination complexes... It does not. $\ce{[Ti(H2O)6]^3+}$ has 13e and $\ce{[Zn(H2O)6]^2+}$ 22e and everything in between exists... Is $\ce{[CuCl4]^2-}$ unstable because it has 17e only? Perhaps better to have 18e $\ce{[CuCl4]^3-}$? $\endgroup$ – orthocresol Aug 26 '16 at 4:27

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