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enter image description here This is from an adapted version of Concise Inorganic Chemistry by J.D Lee.The part on primary valency is confusing me.It mentions that primary valency is the oxidation of the central metal (a number) ,on the other hand I have read that primary valencies are non directional (this description suggests primary valencies are a physical thing).In compounds like $\ce{[Co(H2O)4Cl2]Cl}$ the primary valency is 3 but 2 of the chloride are directional.
Can a ligand be both primary and secondary valency simultaneously?

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I am sorry to see that they are still teaching these concepts of the 19th century as facts. The concept of primary valence and secondary valence is a translation of Werner's terminology (Nobel) as Hauptvalenz and Nebenvalenz. Haupt means something "main" and neben means something "extra, supplementary". What is valence in old school terminology? It is the combining power of an element.

Suppose you have $\ce{Co^{3+}}$, how many counter anions do you expect? $\ce{Cl^-}$? $\ce{CoCl3}$. This is the primary valence of 3. Standard electrostatics, that is, the number of positive charges must equal negative charges. Charged particles cannot stay alone under normal daily conditions.

However, Werner found that compounds of definite composition can also be found such as $\ce{[Co(NH3)6]Cl3}$. But cobalt could only take three chlorides with it? How come there is additional ammonia, that is a part of the molecular "Komplex". He invoked the idea of Nebenvalenz, i.e. supplementary valence to explain the additional ammonia molecules built into this molecule. Indeed this was a complex idea for him and he named it as such Komplexverbindung (complex compound).

I have read that primary valencies are non directional (this description suggests primary valencies are a physical thing).

Delete this from your mind. This does not mean anything today.

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  • $\begingroup$ Ok ,so primary valence is the oxidation state in today's terminology ? $\endgroup$ – Aditya Prakash Jan 24 '20 at 15:56
  • $\begingroup$ Yes, oxidation state=primary valence=Hauptvalenz of Werner. $\endgroup$ – M. Farooq Jan 24 '20 at 16:10
  • $\begingroup$ @M.Farooq: I think the quote in the question is incorrect. This is based on my understanding of a different book (NCERT). Shouldn't the primary valence of the coordination compound be the charge on the complex part (within square brackets) instead of the oxidation state of the central atom? $\endgroup$ – Guru Vishnu May 21 '20 at 13:19

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