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It was only in 1893, that Werner presented a theory known as Werner's coordination theory which could explain all the observed properties of complex compounds. Important postulates of this theory are:
Most elements exhibit two types of valencies:
(a) Primary valency
This corresponds to the oxidation state of the metal ion. This is also called principal, ionisable or ionic valency. It is satisfied by negativeous and its attachment with the central metal ion is shown by lines.
(b) Secondary valency
It is also termed as coordination number of the central metal ion. It is non-ionic or non-ionasable. This is satisfied by either negative ions or neutral molecules. The ligands, which satisfy the coordination number are directly to the metal atom or ion and are shown by thick lines.
Every element tends to satisfy both its primary and secondary valences. In order to meet this requirement a negative ion may often show a dual behavior.
Coordination complex
Although our primary focus in this unit is on bonding, the topic of coordination complexes is so important in chemistry and biochemistry that some of their basic features are worth knowing about, even if their detailed chemistry is beyond the scope of this course. These complexes play an especially crucial role in physiology and biochemistry. Thus heme, the oxygen-carrying component of red blood cells (and the source of the red color) is basically a complex of iron, and the part of chlorophyll that converts sunlight into chemical energy within green plants is a magnesium complex.
Internal sphere
Ligand
K[Ag(CN2)]
Coordination
Number or ligand
external
sphere amins neutral molecule
O H2Oᴼ-aqua
CN-cyanO NH3ᴼ-ammine
complex former Cl-chlorO COᴼ-carbonyl
Br-bromO
SO42-sulphatO
NO3-nitratO
NO2-nitritO
CO32-carbonatO
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