Complexation Reactions and Titrations
Complexation reactions are widely used in analytical chemistry. One of
the earliest uses of these reactions was for titrating cations.
In addition, many complexes are colored or absorb ultraviolet radiation;
the formation of these complexes is often the basis for spectrophotometric
determinations.
Some complexes are sparingly soluble and can be used in gravimetric
analysis or for precipitation titrations as discussed early.
Complexes are also widely used for extracting cations from one solvent
to another and for dissolving insoluble precipitates.
The most useful complex forming reagents are organic compounds
containing several electron- donor groups that form multiple covalent
bonds with metal ions.
Inorganic complexing agents are also used to control solubility, form
colored species, or form precipitates.
The Format ion of Complexes
Most metal ions react with electron-pair donors to form coordination
compounds or complexes. The donor species, or ligand, must have at
least one pair of unshared electrons available for bond formation. Water,
ammonia, and halide ions are common inorganic ligands.
A ligand is an ion or a molecule that forms a covalent bond with a cation
or a neutral metal atom by donating a pair of electrons, which are then
shared by the two.
Coordination number The number of covalent bonds that a cation tends
to form with electron donors.
Typical values for coordination numbers are two, four, and six.
The species formed as a result of coordination can be electrically
positive, neutral, or negative.
For example, copper(II), which has a coordination number of four,
forms a cationic ammine complex, Cu(NH3)4 +2; a neutral complex
with glycine, Cu(NH2CH2COO)2; and an anionic complex with
chloride ion, CuCl4-2
, Complexation Reactions and Titrations
Titrations based on complex formation, sometimes called
Complexometric titrations.
A titration in which the reaction between the analyte and titrant is a
complexation reaction.
The truly remarkable growth in their analytical application, based on a
particular class of coordination compounds called chelates,
A chelate is produced when a metal ion coordinates with two or more
donor groups of a single ligand to form a five- or six- membered
heterocyclic ring.
For example the copper complex of glycineis an example. In this
complex, copper bonds to both the oxygen of the carboxyl group and the
nitrogen of the amine group:
A ligand that has a single donor group, such as ammonia, is called
unidentate
(single-toothed), whereas one such as glycine, which has two groups
available for covalent bonding, is called bidentate.
Tridentate, tetradentate, pentadentate, and hexadentate chelating agents
are also known.
Complexation Equilibria
Complexation reactions involve a metal-ion M reacting with a ligand L to
form a complex ML
Complexation reactions occur in a stepwise fashion and the reaction
above is often followed by additional reactions:
Complexation reactions are widely used in analytical chemistry. One of
the earliest uses of these reactions was for titrating cations.
In addition, many complexes are colored or absorb ultraviolet radiation;
the formation of these complexes is often the basis for spectrophotometric
determinations.
Some complexes are sparingly soluble and can be used in gravimetric
analysis or for precipitation titrations as discussed early.
Complexes are also widely used for extracting cations from one solvent
to another and for dissolving insoluble precipitates.
The most useful complex forming reagents are organic compounds
containing several electron- donor groups that form multiple covalent
bonds with metal ions.
Inorganic complexing agents are also used to control solubility, form
colored species, or form precipitates.
The Format ion of Complexes
Most metal ions react with electron-pair donors to form coordination
compounds or complexes. The donor species, or ligand, must have at
least one pair of unshared electrons available for bond formation. Water,
ammonia, and halide ions are common inorganic ligands.
A ligand is an ion or a molecule that forms a covalent bond with a cation
or a neutral metal atom by donating a pair of electrons, which are then
shared by the two.
Coordination number The number of covalent bonds that a cation tends
to form with electron donors.
Typical values for coordination numbers are two, four, and six.
The species formed as a result of coordination can be electrically
positive, neutral, or negative.
For example, copper(II), which has a coordination number of four,
forms a cationic ammine complex, Cu(NH3)4 +2; a neutral complex
with glycine, Cu(NH2CH2COO)2; and an anionic complex with
chloride ion, CuCl4-2
, Complexation Reactions and Titrations
Titrations based on complex formation, sometimes called
Complexometric titrations.
A titration in which the reaction between the analyte and titrant is a
complexation reaction.
The truly remarkable growth in their analytical application, based on a
particular class of coordination compounds called chelates,
A chelate is produced when a metal ion coordinates with two or more
donor groups of a single ligand to form a five- or six- membered
heterocyclic ring.
For example the copper complex of glycineis an example. In this
complex, copper bonds to both the oxygen of the carboxyl group and the
nitrogen of the amine group:
A ligand that has a single donor group, such as ammonia, is called
unidentate
(single-toothed), whereas one such as glycine, which has two groups
available for covalent bonding, is called bidentate.
Tridentate, tetradentate, pentadentate, and hexadentate chelating agents
are also known.
Complexation Equilibria
Complexation reactions involve a metal-ion M reacting with a ligand L to
form a complex ML
Complexation reactions occur in a stepwise fashion and the reaction
above is often followed by additional reactions:
