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In Concise Inorganic Chemistry (5th Edition) on page 196 , the primary valency of $\ce{[Co(NH3)5Cl]Cl2}$ is given to be 2.

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In other textbooks and in the question here, we get the idea that:

The primary valency of a compound is equal to its oxidation state.

In 'Inorganic Chemistry(4th Edition)' by Catherine E. Housecroft and Alan G. Sharpe':

Werner referred to the oxidation state of the metal ion as its ‘primary valence’ and to what we now call the coordination number as its ‘secondary valence’.

This further suggests that that they are in fact exactly identical and just a matter of how they are referred to.

The oxidation state of cobalt in $\ce{[Co(NH3)5Cl]Cl2}$ is +3 and hence would suggest a primary valency of 3.

What is the correct primary valency of the compound? I am aware that these concepts are not in modern use yet it would be useful to know how Werner originally put forward this idea.

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  • $\begingroup$ Maybe the definition of primary and secondary valences has changed between Werner's time and today. $\endgroup$ – Maurice Oct 4 '20 at 14:03
  • $\begingroup$ I had answered it here: chemistry.stackexchange.com/questions/126954/… $\endgroup$ – M. Farooq Oct 4 '20 at 15:11
  • $\begingroup$ @Maurice . That might be the correct speculation yet it would be beneficial to know exactly how and why it changed. $\endgroup$ – watcher54 Oct 4 '20 at 15:19
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    $\begingroup$ @M.Farooq Thank you. I had already gone through your answer as it is included in the link I attached in the question. I am all for dismissing these concepts as useless in todays context as you mentioned but it still doesnot explain the disprency in the result. $\endgroup$ – watcher54 Oct 4 '20 at 15:22
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    $\begingroup$ I will let you know. Web is full of incorrect info on historical matters and of course on scientific matters too. The work is not in Werner's thesis but rather in his papers in German. They are all online but need subscription. I am basically looking at the book "Coordination Chemistry:A Century of Progress". I am glad that you checked and challenged multiple books. $\endgroup$ – M. Farooq Oct 4 '20 at 15:43
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George B. Kauffman has edited an excellent book, "Coordination chemistry: A century of progress." It was published as an ACS Symposium Series. In the chapter, he writes, "If history teaches us anything, it teaches us that the latest view is not always the best and that change is not always progress." I may add that there is no authority in science. I am glad that you challenged textbooks.

Basically, in Werner's time, the prevalent theory of the complexes was that the complexes are chained compounds, which was not consistent with the solution conductivity and optical isomerism of the complexes. Werner proposed the primary and secondary valence concept.

Contrary to most modern chemistry books, including that respectable Alan G. Sharpe's texts, Werner's primary valence matches the "oxidation" state of the metal in simple cases. The oxidation state matches Werner's primary valence when there is no anion in the coordination sphere. The textbook version of Werner's story fails for the type of compounds you just asked, such as $\ce{[Co(NH3)5Cl]Cl2}$.

Here the oxidation state is indeed 3. However, Werner's primary valency of this compound is 2.

Kauffman mentions

Werner postulated two types of valence—Hauptvalenz, primary or ionizable valence, and Nebenvalenz, secondary or nonionizable valence. According to Werner, every metal in a particular oxidation state, that is, with a particular primary valence, also has a definite coordination number, that is, a fixed number of secondary valences that must be satisfied. Now, whereas primary or ionizable valences can be satisfied only by anions, secondary or nonionizable valences can be satisfied not only by anions but also by neutral molecules containing donor atoms such as nitrogen, oxygen, sulfur, and phosphorus.

The keyword is that primary valence is the ionizable valence. The ionizable valence of the complex in question is 2. $\ce{[Co(NH3)5Cl]Cl2}$ would generate three ions in solution $\ce{[Co(NH3)5Cl]^{2+}}$ and two $\ce{Cl-}$. These experiments were easily doable in Werner's time by conductivity measurements. Secondly, as discussed in the comments, argentometric titrations would show only two chloride ions rather than three.

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