My book (NCERT- Chemistry Part for class 12) Coordination Compounds-NCERT (Page-2)
Mentions the following statements as a postulate of Werner's theory:

The primary valences are normally ionisable and are satisfied by negative ions.

I get that the primary valences basically mean the Oxidation Number of the central metal atom in the coordination sphere. And they are ionisable as the ions that are outside the coordination sphere get dissociated in aqueous solutions.
But is it necessary that Primary Valencies are satisfied only by negative ions?

For example, in $\ce{[Co(NH3)6]Cl3}$ . It is clear that three Chloride ions satisfy the primary valence(+3 oxidation state) of $\ce{Co^{3+}}$ .

But if we consider $\ce{K4[Fe(CN)6]}$ there are no negative ions outside the coordination sphere.
So here are my doubts regarding this

  1. Do the Potassium ions satisfy the primary valence/oxidation state of Iron here? Which means the book is wrong?
  2. If not, then are the Cyanide ions satisfying the primary valence of Iron? If yes, then we can say Ligands can satisfy both Primary and Secondary valences at the same time? (As cyanide ions are already contributing to coordination number(secondary valence))
    But I don't feel this could be true since that contradicts the fact that primary valences are ionisable.

1 Answer 1

  1. Here the primary valence is not satisfied by the potassium ions, it is satisfied by the ligands. Werner's theory says that the primary valence of central atom is only satisfied by anions as the central atom in most complexes are transition metals which have a positive oxidation state.

  2. Yes ligands can satisfy both Primary and Secondary valences at the same time.

According to the pictorial (geometric) representation of Werner's theory, species which satisfy the primary valence are joined by a dashed line, species which satisfy the secondary valence are joined by a solid line and those species which satisfy both valences are joined by a solid and dashed line (in parallel).

So yes ligands can satisfy both the primary and secondary valences.

(You can also refer this video if you understand Hindi.)

  • $\begingroup$ I believe secondary valencies are showed by solid lines as they are the ones which consist of actual bonds between Ligand and Central Metal Atom. Also if Ligands can satisfy both primary and secondary valences, How can we call primary valence as 'Ionisable'? As ligands exist inside Coordination sphere which is not ionisable. $\endgroup$ Commented Apr 11, 2022 at 16:27
  • $\begingroup$ Yes I have fixed that now. Note that all covalent bonds have some ionic character which is why these ligands can provide the primary valence. Werner's theory was not perfect and this ambiguity in the definition of "primary valence" is one of it's many drawbacks. $\endgroup$
    – Anili
    Commented Apr 11, 2022 at 16:49
  • $\begingroup$ Also, other than anions, even water, which is a neutral ligand, acts as a counter ion in some cases through Hydrogen bonding (though an anion is always present), and is present as water of crystallization. Werner's definition of primary valence only being satisfied by anions is wrong and cannot be used in the current day. It would be better for you to learn the concepts of coordination compounds through CFT and VBT, though I am sure you already are given that you are preparing for JEE. $\endgroup$
    – Anili
    Commented Apr 11, 2022 at 16:57

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