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Appendix
2.
Complexes - introduction:
ligands, bonding, co-ordination number and charge on complex ions
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A complex is
formed by the combination of a central
metal ion surrounded by, and bonded to, neutral molecules or ions
acting as 'ligands' (bits stuck on or appendages).
-
A
ligand
is an
atom, ion
or molecule which can act as an electron pair donor (Lewis base) and usually
forms a dative covalent or 'co-ordinate' bond with the central
metal ion.
-
The lone pair
donation is usually from an O, N or halogen atom of the ligand
in this covalent co-ordinate bonding.
-
The central metal
ion acts as a Lewis Acid, that is, an electron pair
acceptor from the ligand by way of vacant 3d, 4s, 4p and even 4d
orbitals for the 3d-block transition elements.
-
The ligand acts as
a Lewis Base, that is, an electron pair donor e.g.
neutral ligands like H2O: (water, aqua in
complex name) or :NH3 (ammonia, ammine in
complex name) and negatively charged ligands like :OH-
(hydroxide, hydroxo in complex name), Cl-
(chloride ion, chloro in complex name) and :CN-
(cyanide ion, cyano in complex name).
-
...
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A an example of
the bonding in a complex ion is shown in the above diagram. The
negative cyanide ion is a monodentate ligand (forms one bond
per ligand) and donates an electron pair into a vacant 3d, 4s or 4p
orbital in the iron(III) ion to form six dative covalent bonds.
-
The resulting
ion has the formula [Fe(CN)6]3-, the
overall charge of 3- is the aggregate of 6- (cyanide ions) plus
3+ (iron ion)
-
The
co-ordination number of 6, which means there are 6 central metal
ion - ligand bonds. It doesn't necessarily mean six ligands, you can
get a co-ordination number of 6 from three co-ordinated bidentate
ligands (2 bonds per ligand), two tridentate ligands and from EDTA
just one ligand can form 6 dative covalent bonds with a central
metal ion. More on this below.
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The most common
complex ion you will come across is the hexaaqua cation of many
metals.
-
It has the
general formula [M(H2O)6]n+
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n, the charge on
the central metal ion and hence the overall charge on the complex
ion n is usually 2 or 3 e.g.
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n = 2 for
titanium(II), vanadium(II), iron(II), cobalt(II), nickel(II),
copper(II) and also the Group 2 alkaline Earth metals magnesium,
calcium etc.
-
and n is 3 for
scandium, titanium(III), vanadium(III), chromium(III), iron(III),
cobalt(III) and also aluminium from Group 3.
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The six
neutral water ligands form 6 dative covalent bonds with the central
metal ion because the bonding pair of electrons comes from
donation of a lone pair from the oxygen atom of the water molecule.
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Therefore the
co-ordination number is 6 and it has a symmetrical octahedral shape.
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The O-M-O bond
angles are all 90o or 180o.
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The ligand may attach
itself by one or more bonds. The suffix '...dentate',
prefixed by mono/uni/bi/ploy/multi e.g. monodentate (unidentate), bidentate, or
polydentate (multidentate)
is used to denote the number of bonds each ligand makes with the
central metal ion.
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The total number of
ligand bonds to the central metal ion is called the co-ordination
number.
-
It is not the number
of ligands, unless it is a monodentate ligand.
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There is no firm
rules relating shape to a particular ligand.
-
The six ligands
don't have to be the same e.g. ...
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... which is the
dichlorotetraaquachromium(III) ion. This octahedral complex with a
co-ordination number of 6, and note this has an overall ion charge of (2
x - from 2Cl-) + (3+ from Cr3+) = +, water is an
electrically neutral ligand.
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Examples of unidentate/monodentate
ligands:
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e.g. neutral ligands:
water H2O:, ammonia :NH3, primary aliphatic
amines e.g. butylamine CH3CH2CH2CH2NH2,
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These ligands
often form octahedral shaped
complexes with a co-ordination number of 6.
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e.g. negative
ligands: chloride Cl-, cyanide CN-,
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The chloride
ion Cl- forms the tetrahedral e.g. the
tetrachlorocuprate(II) complex ion ...
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[CuCl4]2-,
note the overall charge is (2+) + (4 x -) = 2- and the
co-ordination number is 4.
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The chloride ion
can be too bulky to form an
octahedral complex or a square planar complex, though there is no firm rules relating
complex shape to
ligand.
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and CN-
square planar e.g.
the tetracyanonickelate(II) complex ion ...
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[Ni(CN)4]2-,
note the overall charge is (2+) + (4 x -) = 2- and the
co-ordination number is 4.
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Note that [Cu(H2O)4]2+,
the tetraaquacopper(II) ion, with the less bulky water molecule
ligand, forms a blue square planar complex, whereas with the larger
chloride ion, a tetrahedral complex is formed.
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A linear shaped
complex is formed between a silver ion and ammonia.
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[Ag(NH3)2]+
is formed in 'ammoniacal' silver nitrate solution used in the
test for aldehydes. The diamminesilver(I) ion has co-ordination
number of 2 and an overall charge of a single + because the
ammonia molecule is an electrically neutral ligand.
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Examples of bidentate
('two toothed') ligands:
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neutral ligands:
diamines like 1,2-diaminoethane (ethane-1,2-diamine) H2NCH2CH2NH2 (bonds
via lone pair :N).
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negative ligands:
ethanedioate ion C2O42-, (bonds
via lone pair on the :O-). The L represents
where the dative covalent bond forms.
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shows three bidentate ligands co-ordinated to a
central metal ion (co-ordination number 6, 'octahedral' in bond arrangement).
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Examples:
[Cr(H2NCH2CH2NH2)3]3+,
H2NCH2CH2NH2 is
often represented in shorthand by en,
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Bidentate
ligands are the first of what are called polydentate ligands and
such complexes are sometimes called chelates from the
Greek for 'crab's claw' and the complex formation described as a
chelation process.
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More examples of
multi/polydentate ligands:
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EDTA4-
(old name 'EthyleneDiamineTetraAcetic acid') forms six bonds with
a central metal ion and tends to displace all other ligands.
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The haemoglobin
molecule acts as a multi/polydentate ligand with iron(II) ions in
blood chemistry.
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One ligand can replace
another
depending on the relative bond strengths
in a reaction called ligand exchange reaction.
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When a bidentate
or polydentate
ligand is added to a pre-existing complex of monodentate
ligands, it is highly likely
a more stable complex will be formed.
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The principal reason
for this, (ignoring bond strengths), is the positive entropy
change accompanying the 'release' of 4 or 6 small molecules
which offer a greater variation of ways of arranging the particles
or energy
distribution.
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If the ligands are easily
exchanged, the complex is described as 'unstable' and if the ligands
are more strongly bound, the complex would be described as stable.
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Complex ion
stability is also related to the oxidation state of the
transition metal in the presence of a particular ligand.
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See Appendix 3.
for more on complex ion shape and isomerism.
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See Appendix 5.
for more on electrode potentials, oxidation state
and complex ion stability.
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See Appendix 8.
for more on complex ion stability, entropy changes and stability equilibrium constants
(Kstab).

Scandium
* Titanium * Vanadium
* Chromium
* Manganese * Iron * Cobalt
* Nickel
* Copper *
Zinc
* Silver & Platinum
| (spanish) Doc Brown
Química Tabla Periódica Revisión Notas - Parte 10. 3d block
- Transition Metals Bloque 3d - Metales de transición
Appendix 2 Complejos - Introducción: ligandos, la unión, la
coordinación número y carga de iones complejos *
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