hasfau anatolio fabianah
Yogprasad Moneta
Werner was born in 1866 in Mulhouse, Alsace (which was then part of France, but which was annexed by Germany in 1871). He was raised as Roman Catholic.[3] He was the fourth and last child of Jean-Adam Werner, a foundry worker, and his second wife, Salom Jeannette Werner, who originated from a wealthy family.[3] He went to Switzerland to study chemistry at the Swiss Federal Institute (polytechnikum) in Zurich. Still, since this institute was not empowered to grant doctorates until 1909, Werner received a doctorate formally from the University of Zrich in 1890.[3] After postdoctoral study in Paris, he returned to the Swiss Federal Institute to teach (1892). In 1893 he moved to the University of Zurich, where he became a professor in 1895. In 1894 he became a Swiss citizen.[3]
In his last year, he suffered from a general, progressive, degenerative arteriosclerosis, especially of the brain, aggravated by years of excessive drinking and overwork. He died in a psychiatric hospital in Zrich.[3]
Before Werner, chemists defined the valence of an element as the number of its bonds without distinguishing different types of bonds. However, in complexes such as [Co(NH3)6]Cl3 for example, Werner considered that the Co-Cl bonds correspond to a "primary" valence of 3 at long distance, while the Co-NH3 bonds which correspond to a "secondary" or weaker valence of 6 at shorter length. This secondary valence of 6 he referred to as the coordination number which he defined as the number of molecules (here of NH3) directly linked to the central metal atom. In other complexes, he found coordination numbers of 4 or 8.
On these views, and other similar views, in 1904 Richard Abegg formulated what is now known as Abegg's rule which states that the difference between the maximum positive and negative valence of an element is frequently eight. This rule was used later in 1916 when Gilbert N. Lewis formulated the "octet rule" in his cubical atom theory.
In modern terminology, Werner's primary valence corresponds to the oxidation state, and his secondary valence is called coordination number. The Co-Cl bonds (in the above example) are now classed as ionic, and each Co-N bond is a coordinate covalent bond between the Lewis acid Co3+ and the Lewis base NH3.
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.
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.
In hexammine platinic chloride $\ce[Pt(NH3)6]Cl4$ and in potassium ferrocyanide $\ceK4[Fe(CN)6]$ 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 $\ceFe$ 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.
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.
In 1823, Werner put forth this theory to describe the structure and formation of complex compounds or coordination compounds. It is because of this theory that he got the Nobel prize and is known as the father of coordination chemistry. Are you ready to learn the important postulates of this theory?
1) CoCl3.6NH3 Complex: In this compound, the coordination number of cobalt is 6 and NH3 molecules satisfy all the 6 secondary valencies. Chloride ions satisfy the 3 primary valencies. These are non-directional in character. These chloride ions instantaneously precipitate on the addition of silver nitrate. The total number of ions, in this case, is 4, three chloride ions and one complex ion.
2) CoCl3.5NH3 complex: In this compound, cobalt has the coordination number of 6. However, we see that the number of NH3molecule decreases to 5. The chloride ion occupies the remaining one position. This chloride ion exhibits the dual behaviour as it has primary as well as secondary valency.
Werner turned his attention towards the geometrical arrangements of the coordinated groups around the central cation. He was successful in explaining the cause behind optical and geometrical isomerism of these compounds. Some examples are as follows:
1) [CoCl2(NH3)4]Cl complex: According to Werner, there are three structures possible for this complex. These are planar, trigonal prism, octahedral. The number of possible isomers is 3 for planar, 3 for trigonal prism and 2 for octahedral structure.
However, as we could isolate only two isomers of the compound, he concluded that geometrical arrangement of the coordinated group around the central atom in this compound was octahedral. In the case of several other complexes in which the coordination number of the central atom was six, Werner was of the opinion that in all these cases the six coordinated complex have octahedral geometry.
He also read the geometry of the complexes where the coordination number of the central metal atom is 4. He gave two possible structures for such compounds: Square Planar and Tetrahedral. Let us look at an example of the same.
2) [PtCl2(NH3)2] complex: In this complex, the coordination number of the metal is 4. According to Werner, this complex exists in two isomeric forms, cis and trans. This shows that all the four ligands lie in the same plane. Therefore, the structure should be a square planar or tetrahedral.
Alfred Werner's coordination theory was groundbreaking in its time and answered many questions that could not be explained by the theories prevalent then. However, it did have its limitations. Here are the four main limitations of Werner's theory:
It does not explain the bonding in coordination compounds: Werner's theory does not provide any information about the actual nature of the bonds in coordination compounds, i.e., it fails to explain whether the bonds in coordination compounds are ionic, covalent, or coordinate.
It does not explain the difference between strong and weak ligands: Werner's theory treats all ligands on equal footing. It doesn't differentiate between strong field ligands (such as CN-, CO) and weak field ligands (such as F-, Cl-).
It doesn't account for the magnetic behaviour of coordination compounds: The theory cannot explain why some coordination compounds are paramagnetic (attracted to magnetic fields) and others are diamagnetic (not attracted to magnetic fields).
These limitations served as catalysts for the development of more detailed theories such as Crystal Field Theory (CFT) and Ligand Field Theory (LFT) which give more insight into the nature of bond formation and properties of coordination compounds.
Alfred Werner, a Swiss chemist, in 1893 proposed his theory of coordination compounds which is also known as Werner's theory. This theory helped in explaining the structure and bonding in coordination compounds. The main postulates of Werner's theory of coordination compounds are as follows:
Primary valency (also known as ionizable valency) is satisfied by negative ions and is usually non-directional in nature. It indicates the oxidation state of the central metal ion. It's usually satisfied by anions.
Secondary valency (also known as coordination valency or non-ionizable valency) corresponds to the coordination number of the central ion, i.e., the number of ligands attached to the metal ion in the coordination sphere. These are usually satisfied by neutral molecules or ions. This type of valency is directional in nature.
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