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AMHS AP Chemistry
COMPLEX IONS OR COORDINATION COMPLEXES
In chemistry, a coordination complex or metal complex consists of a central atom or ion,
which is usually metallic and is called the coordination center, and a surrounding array of
bound molecules or ions, that are in turn known as ligands or complexing agents.
Ligand:
Many metal-containing compounds, especially those of transition metals, are coordination
complexes.
Examples:
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a
central metal atom to form a coordination complex. The bonding between metal and ligand
generally involves formal donation of one or more of the ligand's electron pairs. The nature of
metal-ligand bonding can range from covalent to ionic. Ligands are viewed as Lewis bases
Lewis Acid/Base Definition:
Definition of a coordinate covalent or dipolar bond: A dipolar bond, more commonly known
as a dative covalent bond or coordinate bond is a kind of covalent bond in which the two
electrons in the bond…
Example of ligands attaching to the metal (and partially displacing the water in the process):
Cu2+(aq) + NH3(aq) ↔ Cu(NH3)2+(aq)
Kf1 = 2.0 x 104
Cu(NH3)2+(aq) + NH3(aq) ↔ Cu(NH3)22+(aq)
Kf2 = 4.7 x 103
Cu(NH3)22+(aq) + NH3(aq) ↔ Cu(NH3)32+(aq)
Kf3 = 1.1 x 103
Cu(NH3)32+(aq) + NH3(aq) ↔ Cu(NH3)42+(aq)
Kf4 = 2.0 x 102
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Cu 2+(aq) + 4NH3(aq) ↔ Caq)
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Knet = 2.1 x 1013
AMHS AP Chemistry
Demo:
200. mL of a 0.10M CuCl2(aq) and add drops of conc. NH4OH followed by drops of conc.
H2SO4
1. Ammonia is a weak base and forms a few ammonium and hydroxide ions in solution
NH3(g) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
2. The hexaaquacopper(II) ions react with hydroxide ions to form a precipitate. This
involves deprotonation of two of the water ligand molecules:
[Cu(H2O)6]2+(aq) (pale blue) + 2OH-(aq) → [Cu(H2O)4(OH)2](s) (pale blue precipitate) + 2H2O(l)
3. The copper(II) hydroxide precipitate reacts with ammonia molecules to form
tetraamminediaquacopper(II) ions This involves ligand exchange:
[Cu(H2O)4(OH)2](s) (pale blue precipitate) + 4NH3(aq) ⇌
[Cu(NH3)4(H2O)2)]2+(aq) (dark blue solution) + 2OH-(aq) + 2H2O(l)
4. Thus the overall reaction, combining 2 with 3, gives:
[Cu(H2O)6]2+(aq) + 4NH3(aq) ⇌ [Cu(NH3)4(H2O)2)]2+(aq) + 4H2O(l)
5. Addition of dilute sulfuric acid introduces H+ ions, which react with NH3 molecules to
form NH4+ ions, and this draws the equilibrium in 4 back to the left-hand side,
regenerating the hexaaquacopper(II) ions in the process.
Example Question:
What is the concentration of Cu2+ ions in a solution prepared by adding 1.00 x 10-3mol of
Cu(NO3)2 to 1.00L of 1.50M NH3?
*Strategy: The Knet is very large…assume the reaction goes to completion to form the complex
ion Cu(NH3)42+ . Thus the initial concentration of this complex ion is limited by the Cu2+ conc.
so is 1.00x10-3M! This then dissociates to produce Cu2+ ions. The Keq for this dissociation is
1/Knet! Simply write the dissociation reaction and solve for the Cu2+ conc.!
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AMHS AP Chemistry
Coordination Chemistry – cont.
 Ligands such as H2O and NH3 which coordinate to the metal via a single coordinate
covalent bond are called monodentate
Dentate?
 Polydentate coordinate with multiple bonds…and polydentate are called chelating ligands
or just chelates
Chelates?
Because two or more bonds need to be broken to separate the ligand from the metal, compounds
with chelated ligands often have greater stability than monodentate ligands.
Practical Examples of Polydentate Ligands in Use:
 Oxalic acid can be used to clean the rust out of water-cooled car engines or steam boilers!
 [EDTA4-] Ethylenediaminetetraacetate ion is a hexadentate ligand and can be added to
commercial salad dressing to remove traces of free metal ions from solution…otherwise
these metal ions can act as catalysts for the oxidation of oils in the dressing…leading to
rancid odors and tastes!
 Hemoglobin – Contains a porphyrin ring which is tetradentate and contains a Fe2+ ion in
the center. A nitrogeneous base from a protein attaches to the other side of the
Fe2+…leaving the sixth coordination position open for an O2 molecule!
This equilibrium reaction is very reversible…especially in the presence of low pH!
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