Because you asked… Here it is, an intro to
Coordination chemistry is the study of compounds formed between metal ions
and other neutral or negatively charged molecules such as Co(NH3)4Cl2+. This is an
example of a metal complex, a charged species consisting of metal ion bonded to
one or more groups of molecules. The bonded molecules are called ligand. The
little picture shown here depicts a structure of a 6-coordinated complex.
*The bonding to the central atom is the same as we learned while studying VSEPR.*
Example: A common metal complex is Ag(NH3)2+, formed when Ag+ ions are mixed with neutral
Ag+ + 2 NH3 -> Ag(NH3)2+
Example: A complex Ag(S2O3)23- is formed between silver ions and negative thiosulfate ions:
Ag+ + 2 S2O32- -> Ag(S2O3)23-
Metal complexes are also called coordination compounds. As in organic chemistry, isomers of
inorganic compounds can exist. (Refer to the Isomers handout for a more thorough explanation
if isomers.) Compounds having the same chemical formula but different structures are called
isomers. Isomers with different geometric arrangements of ligands are geometric isomers.
Isomers whose structures are mirror images of each other are optical isomers. (*Interesting fact,
when a beam of polarized light passes optical isomers or their solutions, the plane of
polarization rotates in opposite directions. The beam rotates to the left for one isomer, and to the
right for its mirror image.*)
Just like an atom and its ion have very different properties, coordination compounds and
coordination complexes are distinct chemical species - their properties and behavior are
different from the metal atom/ion and ligands from which they are composed.
There’s some vocabulary we need to get out
of the way:
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.
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.
Bidentate ligands are Lewis bases that donate two pairs ("bi") of electrons to a metal
Bidentate ligands are often referred to as chelating ligands "Chelate" is derived from the
Greek word for "claw" and sounds like “kee-late.” They can "grab" a metal atom in two
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
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.
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
How are coordination compounds named?
Structures of coordination compounds can be very complicated, and their
names long because the ligands may already have long names. Knowing the
rules of nomenclature not only enable you to understand what the complex
is, but also let you give appropriate names to them.
Nomenclature Part 1: Understand the numbering for ligands. Often, several groups of
the ligands are involved in a complex.
The number of ligand molecules per complex is indicated by a Greek prefix: mono-, di- ,
tri-, tetra-, penta-, hexa, hepta-, octa-, nona-, deca- etc for 1, 2, 3, ... 10 etc.
If the names of ligands already have one of these prefixes, the names are placed in
parentheses. The prefixes for the number of ligands then become bis- (2), tris- (3),
tetrakis- (4), pentakis- (5), hexakis- (6), etc.
Example: Co(en)3Cl3 = tris(ethylenediamine)cobalt(III)chloride)
Nomenclature Part 2: Know your ligands (especially those on bold here – they are the
For neutral ligands, their names are not changed, except the following few:
NH3, ammine (NOTE: two m's, “amine” with 1 m refers to organic compounds)
Normal names that will not change:
NH2CH2CH2NH2, Sometimes Written as “en”, ethylenediamine
The last "e" in names of negative ions are changed to "o" in names of complexes. Often, "ide" is
changed to "o". Note the following:
Cl-, chloride -> chloro
OH-, hydroxide -> hydroxo
O2-, oxide -> oxo
O2- peroxide, -> peroxo
CN-, cyanide -> cyano
N3-, azide -> axido
N3-, nitride -> nitrido
NH2-, amide -> amido
CO32-, carbonate -> carbonato
-ONO2-, nitrate -> nitrato (when bonded through O)
-NO3-, nitrate -> nitro (when bonded through N)
S2-, sulfide -> sulfido
SCN-, thiocyanate -> thiocyanato (-S when bonded through S)
NCS-, thiocyanate -> thiocyanato (-N when bonded through N)
-(CH2-N(CH2COO-)2)2, ethylenediaminetetraacetato (EDTA)
*dashes in front are not negative signs, they indicate the location of a bond*
Nomenclature Part 3: The names of complexes start with the ligands.
Anionic (negatively charged) ligands are listed first, followed with neutral ligands
and then the metal.
The metal complex does not have to carry a charge.
Example: PtCl2(NH3)2 = Dichlorodiammineplatinum
If the complex is negative, the name ends with "ate". End with Roman numerals
representing the oxidation state of the metal.
Example: K[PtCl3NH3] = Potassiumtrichloroammineplatinate(II)
The complex does not have to be negative, it can be positive. If so, it would make
an ionic bond to a cation.
Example: K[PtCl3NH3] = Potassiumtrichloroammineplatinate(II)
Giving Baby a Name:
Example: [Co(NH3)5Cl]Br2, Chloropentaamminecobalt(III) bromide
How do I name this?
In this compound, the metal complex is carrying a 2+ charge, [Co(NH3)5Cl]2+.
You can tell this because of the Bromide anion. [Co(NH3)5Cl]Br2
The Cobalt is carrying a 3+ charge, as the NH3’s (ammine – remember the 2
“m”s!) have no overall charge and the Cl- (chloro) has a 1- charge. Just like in
simpler transition metal compounds, the charge on the metal is the number in the
Now you name this one***: [Cr(H2O)4Cl2]Cl
EDTA – An Inorganic Compound You Know
Some ligands can bond to a metal atom using more than two pairs of electrons. An
example is ethylenediamminetetraacetate ion (EDTA4-), the Lewis structure of
which is shown below. EDTA4- forms very stable complexes with most of the
transition metals. EDTA is a chelating agent.
This hexadentate ligand forms very stable complexes (usually
octahedral structures) with most of the transition metals. The donor
atoms in EDTA4- are the two N atoms, and the four, negatively charged
The sodium salt of EDTA4- (i.e., Na4EDTA) can be found in many commercial products
Soaps, bath products
Beer, food products
EDTA4- is used to "trap" trace amounts of transition metals that could potentially catalyze the
decomposition of a product. Essentially, this acts as a preservative.