Early chemists approached transition metal
complexes using the concept of “valences”
adapted from main group metals.
Metals with a +3 charge, such as iron(III) or
cobalt(III) were believed to make only three
bonds.
A compound such as [Co(NH3)6]Cl3 can form
maximum of three bonds.
Blomstrand in 1869 proposed the chain theory to
explain the structure of metal complexes.
On the basis of assuming only one type of
valancey, Blomstrand and his student Jörgensen
suggested there could be only three bonds to
cobalt(III) in its complexes.
Similarly, Fe in [Fe(H2O)6]Cl2 can make 2 bonds
only
Just before this theory hydrocarbon chain
structures was established. This gave an idea to
Blomstrand to propose chain theory for the
synthesized complexes.
CoCl3.6NH3
CoCl3.5NH3
CoCl3.4NH3
CoCl3.3NH3
Each Complex has 3 Chlorides, and we know that
Cl- can precipitate with AgNO3
Complex Number of chloride Present formulation
ions precipitated
CoCl3.6NH3 3 [Co(NH3)6]3+ , 3Cl-
CoCl3.5NH3 2 [Co(NH3)5Cl]2+ , 2Cl-
CoCl3.4NH3 1 [Co(NH3)4Cl2]+ , Cl-
IrCl3.3NH3 0 [Ir(NH3)3Cl3]
Solution of lrCl3.3NH3 or [Ir(NH3)3Cl3] revealed negative results for
i)electrical conductivity measurement and ii)precipitation (upon
addition of AgNO3 solution, precipitate did not form).this was
Jorgensen success to prove his incorrectness of chain theory.
Complex Molar Number of ions Present formulation
conductivity indicated
Ohm -1
PtCl4.6NH3 523 5 [Pt(NH3)6]4+ , 4Cl-
PtCl4.5NH3 404 4 [Pt(NH3)5Cl]3+ , 3Cl-
PtCl4.4NH3 229 3 [Pt(NH3)4Cl2]2+ , 2Cl-
PtCl4.3NH3 97 2 [Pt(NH3)3Cl3]+ , Cl-
PtCl4.2NH3 0 0 [Pt(NH3)2Cl4]
PtCl4. NH3.KCl 109 2 K+, [Pt(NH3)Cl5]-
PtCl4. 2KCl 256 3 2K+, [PtCl6]2-
• Three ammonia molecules, linked to metal
directly by the nitrogen atoms and
alternatively linking / connecting three
chlorides to the metal.
• Being at a certain distance from metal, the
chloride ions will be precipitated upon
addition of Ag+ ion of solution(AgNO3)
Two ammonia molecules linked to metal by nitrogen
atom, alternatively connecting two chlorides to the
metal by ammonia molecule chains.
Being at a certain distance from metal, these two
chloride ions will be precipitated upon addition of Ag+
ion solution(AgNO3).
One chloride is directly linked with metal so this
chloride ion will not ionize and precipitate as silver
chloride.
One ammonia molecule linked by nitrogen
atom, alternatively the ammonia chain is
connecting one chloride to the metal.
Two chlorides are strongly linked to metal
directly. Hence supposed as non-ionizable
chlorides.
Its would show the similar behavior as
CoCl3.4NH3, after the addition of
precipitating agent.
Alfred Werner, professor of chemistry in Zurich
and winner of a Nobel prize in 1913.
In 1893, at age 26, he proposed what is now
commonly referred to as “Werner’s
coordination theory”
Jorgensen supported Blomstrand’s approach,
and Werner, in order to support his theory,
synthesized new compounds and studied their
isomers. Eventually, in 1907, Werner prevailed
and proved the octahedral geometry of
coordination compounds
Three of its most important postulates are:
I) Most elements exhibit two types of valencies:
(a) Primary Valancy (represented by dashed line,
----
(b) Secondary valency (represented by a solid
line ).
In modern terminology,
Primary valency corresponds to oxidation state
Secondary valency corresponds to coordination
number.
II)Every element tends to satisfy both its
primary and secondary valencies.
III) The secondary valency is directed toward
fixed positions in space
(NOTE: The Primary Valency is ionizable and
nondirectional
whereas
the secondary valency is nonionizable and
directional)
Metals have two valancies
Primary Secondary
corresponds to corresponds to
oxidation state Coordination number
It ranges from 2 to 8
+ve -ve neutral While 4 & 6 are most
common
Experimental facts and
Werner’s Theory
(in upcoming slides)
Alfred Werner proposed that the ammonia
molecules could bond strongly and directly
to the metal
While chlorides either directly bonded, or
bonded after NH3
Any Chloride next to NH3 is ionic in solution.
Primary valancy or Oxidation No
is 3, neutralized by chlorides
Secondary valancy
or Coordination No.=6
Satisfied by six ammonia
All are inside the coordination sphere,
surrounding the metal ion Co(III)
chloride ions does not serve as ligands here.
All these Chloride ions will readily precipitated
as AgCl.
Explanation of postulate (II)
Both the primary and secondary valance tend
to be satisfied.
As only five NH3 are there to satisfy the
secondary valancy, therefore one Cl- ion has
to serve dual function here to satisfy primary
and secondary valancies. Now represented as
[Co(NH3)5Cl]Cl2, the one chloride ion
represented as will not readily precipitated
by Ag+.
Two chloride ions are satisfying both the
valancies
In solution the compound will dissociates
into two ions [Co(NH3)4Cl2]+ and Cl-.
Werner theory predicted that
this compound will not yield any
Cl- ion in solution.
None of the Chlorides in the compound
[M(NH3)3Cl3] are ionizable
(Note: M = Co(cobalt), Ir (irridium) etc.)
Coordination theory correctly explains
many of the structural features of
coordination compounds.
Specifically the structures of complexes
with coordination number 6 were
determined.
It is the system 6 ligands surrounds the
central metal atom.
Three of the more probable structures
for six ligands metal complexes
assumed were
Hexagonal planar
Trigonal prism
octahedral
This approach correctly predicted that there
would be two forms of CoCl3.4NH3
General Chemistry by Hill Petrucci
General Chemistry by Brady and Holumn
General Chemistry by Daniel Harris
Coordination Chemistry by Basolo and
Johnson
Inorganic Chemistry by Kettle
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