According to the Werner's theory of co-ordination compounds.
1) Metals possess two types of valencies:
a) Primary valency or ionizable valency. It is also referred to oxidation state.
b) Secondary valency which a metal atom or cation exercises towards neutral molecules or negative groups (ligands) in the formation of complex ions.The secondary valency is also called the coordination number.
Example: In [Pt(NH3)6]Cl4 secondary valency of Pt is 6
2) Primary valencies are satisfied by negative ions, secondary valencies may be satisfied by negative ions or neutral molecules.
3) Ligands satisfying secondary valencies are directed towards fixed positions in space giving a definite geometry to the complex, but the primary valencies are non-directional.
Six valencies are directed towards a regular octahedron while four are directed towards either a tetrahedral manner or square planar.
1) Metals possess two types of valencies:
a) Primary valency or ionizable valency. It is also referred to oxidation state.
b) Secondary valency which a metal atom or cation exercises towards neutral molecules or negative groups (ligands) in the formation of complex ions.The secondary valency is also called the coordination number.
Example: In [Pt(NH3)6]Cl4 secondary valency of Pt is 6
2) Primary valencies are satisfied by negative ions, secondary valencies may be satisfied by negative ions or neutral molecules.
3) Ligands satisfying secondary valencies are directed towards fixed positions in space giving a definite geometry to the complex, but the primary valencies are non-directional.
Six valencies are directed towards a regular octahedron while four are directed towards either a tetrahedral manner or square planar.
Before Werner, chemists defined the valence of an element as the number of its bonds without distinguishing different types of bond. 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 distance. 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.
Today Werner's primary valence corresponds to the oxidation state, and the secondary valence is always 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.
Coordination Compound
A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions (called ligands) bond to a central metal atom (or ion) by coordinate covalent bonds. Coordination compounds and complexes are distinct chemical species - their properties and behavior are different from the metal atom/ion and ligands from which they are composed.
A coordination compound is formed when groups of atoms, ions, or molecules chemically bond with each other by donating and accepting pairs of electrons. Groups donating electron pairs are called ligands. They are usually Lewis bases. Groups accepting electron pairs are often transition metal cations.
They are usually Lewis acids. Chemical bonds formed in this way are called coordinate-covalent, or dative bonds. As in any covalent bond, two electrons are shared between transition metal and ligand. But in a coordination compound, both electrons come from a pair found on the ligand .
The metal cation simply acts as the electron pair acceptor, itself donating no electrons to the bond. Because of the complicated nature of these arrangements, coordination compounds are often called coordination complexes or simply complexes. While nearly all cations can form coordination compounds with ligands. Ligands come in all shapes and sizes, though they are usually non-metals from the right side of the periodic table.
A coordination compound is formed when groups of atoms, ions, or molecules chemically bond with each other by donating and accepting pairs of electrons. Groups donating electron pairs are called ligands. They are usually Lewis bases. Groups accepting electron pairs are often transition metal cations.
They are usually Lewis acids. Chemical bonds formed in this way are called coordinate-covalent, or dative bonds. As in any covalent bond, two electrons are shared between transition metal and ligand. But in a coordination compound, both electrons come from a pair found on the ligand .
The metal cation simply acts as the electron pair acceptor, itself donating no electrons to the bond. Because of the complicated nature of these arrangements, coordination compounds are often called coordination complexes or simply complexes. While nearly all cations can form coordination compounds with ligands. Ligands come in all shapes and sizes, though they are usually non-metals from the right side of the periodic table.
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