The chemistry of organic nitrogen (organonitrogen) compounds
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Formation of transition metal ion (3d block) complexes with primary
The formation of ammine complexes
with transition metal ions
(a) A brief introduction
Amines can act as electron pair donor
ligands with transition metal ions of the d-block e.g. the 1st
transition metals series ions of chromium, cobalt, nickel and copper.
The lone pair of electrons of the nitrogen of the
amine group can form a dative covalent bond with an empty orbital of the
central metal ion e.g. Cr3+, Co2+, Ni2+,
Cu2+ and other transition metal ions.
The reactions are very similar to that between these
ions and ammonia i.e. an initial metal hydroxide complex precipitate
that dissolves in excess amine to give (usually) a deep blue soluble
For more details on complexes see:
Introduction to the chemistry of
complexes ligands, chemistry
of chromium, chemistry
chemistry of nickel
(b) Complexes with primary aliphatic
amines - monodentate ligands
A monodentate ligands have one group with a non-bonding
lone pair of electrons that can act as a lone pair donor to form a
dative covalent bond with the central metal ion.
In this case the lone pair of electrons on the
nitrogen atom of an aliphatic amine.
reaction of copper(II)
ions and ammonia
start with the more familiar inorganic example of complexing copper(II)
ions with ammonia. On adding ammonia solution to a copper(II) sulfate
solution, after an initial copper(II) hydroxide precipitate, with excess ammonia, a
blue solution is formed of the ammine complex ion (ligand substitution is
incomplete), the overall changes can be expressed as:
Initially a 'turquoise' precipitate
of the hydrated copper(II) hydroxide is formed.
+ 2OH–(aq) + 2H2O(l)
Then the hydroxide precipitate
dissolves in excess ammonia to give the soluble deep blue complex ion.
The overall change as
You can then repeat the experiment with solutions of
aliphatic amines e.g. ethylamine(aq)
complexes with aliphatic amines as the ligand
aqueous solution of an aliphatic amine is added to a copper(II) salt
solution containing the hexaaquacopper(II) complex ion, an initial
neutral hydrated 'turquoise'/pale blue coloured copper(II) hydroxide complex precipitate is formed,
Then, with excess amine, the copper(II)
hydroxide precipitate dissolves and a deep
blue solution is formed of the ammine complex ion, equation (ii).
The changes can be expressed as:
+ 2OH–(aq) + 2H2O(l)
and the overall reaction is
These are NOT redox reactions, but
ligand exchange substitution reactions with no change in oxidation
Both copper(II) complexes are
octahedral, co-ordination number 6, overall charge remains at 2+
because both ligands are electrically neutral.
The complexes with an amine RNH2
(R = alkyl), ethylamine (CH3CH2NH2) and
ammonia (NH3) are shown below.
You get similar reactions with other
transition metal ions e.g. the hexaaqua cobalt(II) ions, nickel(II) ions
and chromium(III) ions. which all give hydroxide precipitates that dissolve in excess aqueous aliphatic
amine solution to give soluble blue transition metal complexes.
The formula of the neutral
hydroxide precipitates would be:
and the formula of the final soluble complexes
formed from aliphatic primary amines would be
where R = alkyl i.e. for
ethylamine R = CH3CH2 (if R = H, the ligand is
(c) Complexes with
primary aliphatic amines - polydentate ligands
Polydentate ligands have at least two groups with a
non-bonding lone pair of electrons that can both act as a lone pair donors to form
a dative covalent bond with the central metal ion.
A classic bidentate ligand (two
electron pair donor groups in the same molecule) is 1,2-diaminiethane.
Structural formula H2NCH2CH2NH2
Please note, it is common to signify this molecule as
'en', shorthand for its old name of ethylenediamine.
I have also used the abbreviation
L-L to signify a bidentate ligand such as 1,2-diaminoethane.
The diagram on the right shows three
bidentate ligands coordinated to a
central metal ion. The coordination number is still 6 (NOT 3), and
the bonds do form an 'octahedral' arrangement around the central
transition metal ion.
Hexaaqua Cr(III) ions, Ni(II) ions,
Co(II) ions and Cu(II) form this type of complex.
'en' or 'L-L' to represent 1,2-diaminoethane, you can
write the complex formations as an overall ligand exchange reaction
The diagram shows the structure of
the nickel complex and note it exhibits R/S isomerism!
This structure is common to many transition metal ion - bidentate ligand
showing mirror image forms of the complex.
Note the copper(II) complex can involves
just two ligands in a square planar configuration.
(Not sure if Cu2+ complexes
with 3 molecules of 1,2-diaminoethane?)
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