What Is A Coordination Compound?
A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules
or anions (called ligands) bond to a central metal atom (or ion) by coordinate covalent bonds.
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Ligands are Lewis bases - they contain at least one pair of electrons to donate to a metal
atom/ion. Ligands are also called complexing agents.
Metal atoms/ions are Lewis acids - they can accept pairs of electrons from Lewis bases.
Within a ligand, the atom that is directly bonded to the metal atom/ion is called the donor
atom.
A coordinate covalent bond is a covalent bond in which one atom (i.e., the donor atom)
supplies both electrons. This type of bonding is different from a normal covalent bond in
which each atom supplies one electron.
If the coordination complex carries a net charge, the complex is called a complex ion.
Compounds that contain a coordination complex are called coordination compounds.
Coordination compounds and complexes are distinct chemical species - their properties and
behavior are different from the metal atom/ion and ligands from which they are composed.
The coordination sphere of a coordination compound or complex consists of the central metal
atom/ion plus its attached ligands. The coordination sphere is usually enclosed in brackets when
written in a formula.
The coordination number is the number of donor atoms bonded to the central metal atom/ion.
Some Coordination Complexes
example
molecular
formula
Lewis
base/ligand
Lewis
acid
donor
atom
coordination
number
[Ag(NH3)2]+
NH3
Ag+
N
2
[Zn(CN)4]2-
CN-
Zn2+
C
4
[Ni(CN)4]2-
CN-
Ni2+
C
4
[PtCl6]2-
Cl-
Pt4+
Cl
6
[Ni(NH3)6]2+
NH3
Ni2+
N
6
Common Structures
Why do we want to know the structure for a coordination compound? The compound's structure
(i.e., how the ligands are arranged around the metal atom) determines its physical and chemical
properties. For example, a tetrahedral compound will behave differently than a square planar
compound that contains the same metal atom and the same ligands.
Consider the complex ion, [CoCl4]2-. The coordination number of [CoCl4]2- is equal to 4; thus,
the structure of [CoCl4]2- might be:
1. tetrahedral - ligands attached at the corners of a tetrahedron
2. square planar - ligands attached at the corners of a square
3. something else?
Coordination compounds with a coordination number (CN) of 4 are generally either tetrahedral
or square planar. The [CoCl4]2- complex ion is experimentally known to have a tetrahedral
structure.
A Comparison of Tetrahedral and Square Planar Structures
example
sketch
model
[CoCl4]2tetrahedral
CN = 4
ligands attached at the corners of a
tetrahedron
[Ni(CN)4]2square planar
CN = 4
ligands attached at the corners of a square
Coordination numbers range from 1 to 12, with 2, 4 and 6 being the most common. Linear and
octahedral are the most common structures for coordination numbers 2 and 6, respectively.
A Comparison of Linear and Octahedral Structures
example
[Ag(NH3)2]+
linear
CN = 2
ligands and metal atom connected in a
straight line
sketch
[PtCl6]2octahedral
CN = 6
ligands attached at the corners of an
octahedron
Structures With Monodentate Ligands
Monodentate ligands are Lewis bases that donate a single pair ("mono") of electrons to a metal
atom. Monodentate ligands can be either ions (usually anions) or neutral molecules.
Some Monodentate Ligands
ligand
Lewis structure
name
ligand
Lewis structure
name
F-
fluoride
ion
Cl-
chloride ion
Br-
bromide
ion
I-
iodide ion
H2O
water
NH3
ammonia
OH-
hydroxide
ion
CO
carbon
monoxide
CN-
cyanide
ion
SCN-
thiocyanate
ion
Chemists often represent ligands as spheres for simplicity, even though the "sphere" sometimes
has three-dimensional structure of its own. For example, when chemists draw the structure for
[Ni(NH3)6]2+, each ammonia ligand is represented as a sphere. The sphere represents the donor
atom of the ligand. In [Ni(NH3)6]2+, the donor atoms are the nitrogen atoms of the NH3 ligands
(NOT the hydrogen atoms).
= Ni
=N
=H
[Ni(NH3)6]2+
abbreviated structure
Note: only the donor atoms
(N atoms) of the NH3
ligands are shown.
[Ni(NH3)6]2+
complete structure
Note: all atoms are shown.
Applications
The dark-blue, square planar [Cu(NH3)4]2+ complex ion is present in some brands of waterbed
conditioners. It is responsible for inhibiting the growth of fungi and bacteria. The square planar
[RhI2(CO)2]- complex ion is used as a catalyst in the "Monsanto Process" for making acetic acid,
the active ingredient in vinegar.
= Cu/Rh
=N
=H
=O
=C
=I
[Cu(NH3)4]2+
Used in some brands of
waterbed conditioners to
inhibit the growth of fungi
and bacteria. Note the
square planar structure.
[RhI2(CO)2]Used as a catalyst in the
Monsanto Process for
making acetic acid. Note the
square planar structure.