Chain theory assumes that the ammonia groups
in metal ammines are bonded together in chains.
One example would be Co(NH3)5Cl3,
a typical metal ammine.
One chain theory structure for this compound would be:
This structure agrees with the results of silver
precipitation experiments. It also involves a chain of four ammonia
groups, which Jørgensen
regarded as a particularly stable arrangement.
This is a sensible assumption. The description of organic
molecules as chains of CH2 groups
has been very successful. For instance, C4H10 is
formulated as:
CH3-CH2-CH2-CH3
or
You might notice that chain structures give nitrogen
five bonds, -NH3-. This looks
unusual to us. You must remember that 19th century chemists
did not know that NH4Cl was
ionic, so that is appeared as if nitrogen has a valence
of five. The metal ammine chain structures look very
much like organic chains:
-NH3-NH3-NH3-NH3-
-CH2-CH2-CH2-CH2-
Jørgensen observed
that some chlorines were more easily removed than others.
In the ammine shown, two Cl's
are
precipitated by silver nitrate--the third Cl is more tightly
held. The easily removed Cl's were assumed to be attached
to ammonia, the tightly held Cl to the metal.
