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Werner’s Theory and Isomerism:

Werner turned his attention towards the geometrical arrangements of the coordinated groups around the central cation and explained successfully the cause of optical and geometrical isomerism of these compounds. Some examples are given below:
Octahedral complexes:
[CoCl2(NH3)4]Cl: Werner said that theoretically there are three structure possible for this complex. These are planar, trigonal prism, octahedral. The number of possible isomer is for planar, 3 for trigonal prism and 2 for octahedral structure.
Various possible isomers for the planar, trigonal prism and octahedral structures of the complex ion [CoCl3(NH3)4] since only two isomers of the compound could be isolated, werner 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 able to conclude that in all these cases the six coordinated complex have octahedral geometry.
Square Planar and Tetrahedral:
He studied the geometry of the complexes in which the coordination number of the central metal atom is 4. He proposed that there are two possible structures. 
 [PtCl2(NH3)2] complex: In this complex the coordination number of the metal is 4, werner found that it existed 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
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Cis and trans isomers of [PtCl2(NH3)2] complex

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