In coordination
complex central atom metals exert two types of valencies; The important
postulates of Werner’s theory are as follows:
(1) The primary or ionizable valencies
which are satisfied by negative ions and equal the oxidation state of the central
metal atom.
(2) The Secondary or non ionizable valencies
which can be satisfied by neutral or negative ions/groups (or sometimes by cationic species -discovered
later).
(3) The secondary valencies equal the
coordination number of central metal atom/ion. This number is fixed for a
metal.
(4) The ions/groups bound by the secondary
linkages have characteristic spatial arrangements corresponding to different
co-ordination numbers. In the modern terminology, such spatial arrangements are
called coordination polyhedra. The various possibilities are
Coordination
Number = 2 Linear
Coordination
Number = 3 Triangular
Coordination
Number = 4 Tetrahedral or sq. planar
Coordination
Number = 5 square pyramidal or TBP
Coordination
Number = 6 Octahedral
(5) The secondary valencies are generally
represented by solid lines while the primary valencies are represented by
dashed lines and the ions which satisfy both primary and secondary valencies
will be drawn with both solid and dashed lines.
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