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Many metal ions (usually transition metals but including a few others) form
complex ions with a variety of anions and molecules. These are formed in
a similar way as any other molecule is: for example:
- Fe+3(aq) + 6CN-(aq) ->
Fe(CN)6-3(aq)
- and
- Cu+2(aq) + 4NH3(aq) ->
Cu(NH3)4+2(aq)
The difference here is
that the electron pair that forms the bond starts as a lone pair on the molecule
or ion. In this process the metal ions are acting as Lewis acids:
they are electron pair acceptors, binding to the lone pairs of electrons on the
cyanide ion and ammonia molecule.
There is a bit of unique nomenclature to complex ions:
- The metal is known as the central metal ion
- The anions or molecules attached to the metal are called ligands
- The coordination number is the number of places on the metal ion
where ligands are bound.
- The bond between the metal ion and the ligand, where the ligand supplies
both electrons, is known as a coordinate covalent bond
Most metal ions exist as water complexes in water solution. For example,
Cu+2 is not a bare ion in water: rather, it exists as the
Cu(H2O)6+2 ion.
Example: What is the central metal ion, ligand and coordination number
in the Cu(H2O)6+2 ion?
Solution: The central metal ion is Cu+2. The
ligands are water molecules, and since there are six water molecules
bound to the ion, the coordination number is 6.
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