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According to Werner's theory, metal ions have two types of valency-primary and secondary, where the primary valency is said to be satisfied by negative ions only, and the secondary valency can be satisfied by positive ligand, negative or neutral molecule.

My question here is: While formation of complex compounds, which of these valencies is satisfied?

What I'm not able to interpret from this is whether there is a 'possibility' for both the valencies being satisfied or is it necessary that both are satisfied?

Also if the later is true how is it possible? Can it be explained taking some example as reference.

I think I'm not able to interpret it and understand its details.

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A certain number of ions, atoms or molecules closely associate around a central atom leading to the formation of distinct entity called coordination complex. The groups atoms or molecules linked to the central atom are said to be coordinated with the latter and their number gives the coordination number.

The non-ionisable units lie in the primary zone or sphere, whereas the ionisable units lie in the secondary sphere and appear as ions in the solution.

In hexammine platinic chloride $\ce{[Pt(NH3)6]Cl4}$ and in potassium ferrocyanide $\ce{K4[Fe(CN)6]}$ four chlorine ions of the former and four potassium ions of the latter ionize in the aqueous solution, whereas six ammonia molecules of the former and six cyanogen groups of the latter belong to the primary sphere or non-ionizable sphere and thus fail to respond to their characteristic tests or fail to ionize. Or you could also say that the chlorine ions in the former and the potassium ions in the latter occupy the primary valencies and hence are ionizable and the ammonia and cyanogen group occupy the secondary valencies and are non-ionizable, while the $\ce{Fe}$ atom in the latter is present in the primary Sphere and thus fails to ionize.

As per your question during the formation of a compound say $\ce{[Pt(NH3)5Cl]Cl3}$, five molecules of ammonia and the chlorine ion (linked by secondary valencies) are present in the primary Sphere and the rest of the three chlorine atoms lie in the secondary sphere. Thus the coordination number of the compound would be six as five ammonia molecules and one chlorine atom are attached to the central metal atom.

Now, every element tends to satisfy both its primary and secondary valencies. A negative ion tends to satisfy both these valencies i.e. primary as well as the secondary valencies. The presence of a negative ion in the coordination sphere reduces the amount of charge on the complex ion by the amount of charge present on it while the negative ions in the primary sphere or the coordination sphere are not ionized.

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    $\begingroup$ Thanks this was a great help. But I still have a doubt and that is: according to werner's theory primary valency can be satisfied only by negative ions, but in many cases just as in potassium ferrocyanide, it is satisfied by a positive ion, isn't it? Then how is this explained? Also the last part of your answer would be true only if the complex ion would be carrying a negative charge, right? Please correct me if I'm wrong. $\endgroup$ – Shefali Jan 3 '14 at 2:32
  • $\begingroup$ Primary Valency: It is the no. of charges present on the complex ion.Usually a complex ion possess positive charge. And thus negative ions satisfy this charge. But as in the cases of potassium ferrocyanide the the charge on the coordination sphere is negative owing to the -1 charge of each CN ion. Here the charge on iron would be +2 and thus the complex as a whole would posses -4 charge which is balanced by four potassium ions. $\endgroup$ – user1811 Jan 3 '14 at 3:17
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Werner's theory proposed by alfred werner in 1898.He was awarded by noble prize in 1913. It explains about the formation of coordinate complex compounds, bonding of coordinate complex compounds, stability of coordinate complex compounds and isomerism of coordinate complex compounds.

According to Werners theory each coordinate complex compound exhibits two types of valencies.The are a) primary valency and b) secondary valency. Primary valencies are satisfied by only negative charge and it is equal to oxidation state of central metal ion.

Secondary valencies are satisfied by either negative charge or neutral species and it is equal to the coordinate number. Primary valencies are denoted by dotted lines and ionisible. Secondary valenices are denoted by dark or straight lines and non ionisible.

In coordinate complex compound formed bonds are coordinate covalent bonds. In coordinate complex the central metal atoms acts as the Lewis acid and the ligand acts as the Lewis base. If coordinate complex shows six ligand in it then the structure is octahydral and four ligand in it then the structure is tetrahydral.

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  • $\begingroup$ Well... while almost all six-coordinate complexes are octahedral, not all four-coordinate complexes are tetrahedral. Many are square planar. $\endgroup$ – Geoff Hutchison Dec 16 '15 at 17:30

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