Coordination chemistry is the study of compounds formed between metal ions and other neutral or
negatively charged molecules. The compound is known as a metal complex, which is a charged
species consisting of metal ion bonded to one or more groups of molecules. The bonded molecules
are called ligands.
Their structures are important with respect to data and properties. Compounds having the same
chemical formula but different structures are called isomers.
Isomers with different geometric arrangements of ligands are called geometric isomers
Isomers whose structures are mirror images of each other are called optical isomers. When a
beam of polarized light passes through optical isomers or their solutions, the plane of
polarization rotates in different directions. The beam rotates to the left for one isomer, and right
for its mirror image.
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
NH2-groups are bound to an asymmetric alpha carbon
β N-C-C-COOH γ N-C-C-C-COOH
20 different chemical (R) groups make the common amino acids &
20 ubiquitous & universal amino acids are found in all living systems
every amino acid (but one) exists in two isomeric molecular forms:
each with different arrangements of atoms in space
i.e., 2 optical isomers or mirror images of each other (left-right handed*)
molecules that exhibit handed-ness are said to be CHIRAL
and the mirror images are referred to enantiomers
but only the L-optical isomer amino acids occur in biological proteins
... an anomaly of molecular evolution?
The coordination chemistry was pioneered by Nobel Prize winner Alfred Werner (1866-1919). He
received the Nobel Prize in 1913 for his coordination theory of transition metal-amine complexes.
At the start of the 20th century, inorganic chemistry was not a prominent field until Werner studied
the metal-amine complexes such as [Co(NH3)6Cl3].
Werner recognized the existence of several forms of cobalt-ammonia chloride. These compounds
have different color and other characteristics. The chemical formula has three chloride ions per
mole, but the number of chloride ions that precipitate with Ag+ ions per formula is not always
three. He thought only ionized chloride ions will form precipitate with silver ion.
Proposed Structure of Cobalt Ammonia Complexes from Number of Ionized Chloride
Solid Color Ionized Cl- Complex formula
CoCl36NH3 Yellow 3 [Co(NH3)6]Cl3
CoCl35NH3 Purple 2 [Co(NH3)5Cl]Cl2
CoCl34NH3 Green 1 trans-[Co(NH3)4Cl2]Cl
CoCl34NH3 Violet 1 cis-[Co(NH3)4Cl2]Cl
The structures of the complexes were proposed based on a coordination sphere of 6. The 6
ligands can be ammonia molecules or chloride ions. Two different structures were proposed for
the last two compounds, the trans compound has two chloride ions on opposite vertices of an
octahedral, whereas the two chloride ions are adjacent to each other in the cis compound. The cis
and trans compounds are known as geometric isomers.
Structures of coordination compounds can be very complicated, and their names are long
because the ligands may already have long names. Knowing the rules of nomenclature not only
enables you to understand what the complex is, but also lets you give appropriate names to them.
The coordination compounds are named in the following way:
A. To name a coordination compound, no matter whether the complex ion is the cation or the anion, always
name the cation before the anion. (This is just like naming an ionic compound.)
B. In naming the complex ion:
1. Name the ligands first, in alphabetical order, then the metal atom or ion.
Note: The metal atom or ion is written before the ligands in the chemical formula.
2. The names of some common ligands are listed in Table 1.
A. -o"; for anions that end in "-ide"(e.g. chloride), "-ate" (e.g.
sulfate, nitrate), and "-ite" (e.g. nirite), change the endings as follows: -ide -o; -ate -ato;
(ethylenediamine). Important exceptions: water is called ‘aqua’, ammonia is called ‘ammine’,
carbon monoxide is called ‘carbonyl’, and the N2 and O2 are called ‘dinitrogen’ and ‘dioxygen’.
Table 1. Names of Some Common Ligands
Anionic Names Neutral Ligands Names
Br- bromo NH3 ammine
F- fluoro H2O aqua
O2- oxo NO Nitrosyl
OH- Hydroxo CO Carbonyl
CN- cyano O2 dioxygen
C2O42- oxalato N2 dinitrogen
CO32- carbonato C5H5N pyridine
CH3COO- acetato H2NCH2CH2NH2 ethylenediamine
3. Greek prefixes are used to designate the number of each type of ligand in the complex ion,
a) e.g. di-, tri- and tetra-.
b) If the ligand already contains a Greek prefix (e.g. ethylenediamine) or if it is polydentate ligands
(ie. can attach at more than one binding site) the prefixes bis-, tris-, tetrakis-, pentakis-, are used instead.
Table 2. Numerical Prefixes
Number Prefix Number Prefix Number Prefix
1 mono 5 penta 9 nona
2 di (bis) 6 hexa 10 deca
3 tri (tris) 7 hepta 11 undeca
4 tetra 8 octa 12 dodeca
4. After naming the ligands, name the central metal.
a) If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex
cation is called cobalt and Pt is called platinum.
b) If the complex ion is an anion, the name of the metal ends with the suffix –ate. For example, Co in a
complex anion is called cobaltate and Pt is called platinate. For some metals, the Latin names are used in the
complex anions e.g. Fe is called ferrate (not ironate).
Table 3: Name of Metals in Anionic Complexes
Name of Metal Name in an Anionic Complex
5. Following the name of the metal, the oxidation state of the metal in the complex is given as a Roman numeral
Examples Give the systematic names for the following coordination compounds:
Answer: triamminetriaquachromium(III) chloride
Solution: The complex ion is inside the parentheses, which is a cation.
The ammine ligands are named before the aqua ligands according to alphabetical order.
Since there are three chlorides binding with the complex ion, the charge on the complex ion must be +3
(since the compound is electrically neutral).
From the charge on the complex ion and the charge on the ligands, we can calculate the oxidation number
of the metal. In this example, all the ligands are neutral molecules. Therefore, the oxidation number of
chromium must be same as the charge of the complex ion, +3.
Answer: pentaamminechloroplatinum(IV) bromide
Solution: The complex ion is a cation, the counter anion is the 3 bromides.
The charge of the complex ion must be +3 since it bonds with 3 bromides.
The NH3 are neutral molecules while the chloride carries - 1 charge. Therefore, the oxidation number of
platinum must be +4.
Answer: dichlorobis(ethylenediamine)platinum(IV) chloride
Solution: ethylenediamine is a bidentate ligand, the bis- prefix is used instead of di-
Answer: potassium hexacyanoferrate(II)
Solution: potassium is the cation and the complex ion is the anion.
Since there are 4 K+ binding with a complex ion, the charge on the complex ion must be 4.
Since each ligand carries –1 charge, the oxidation number of Fe must be +2.
The common name of this compound is potassium ferrocyanide.
Answer: sodium tetrachloronickelate(II)
Solution: The complex ion is the anion so we have to add the suffix –ate in the name of the metal.
Electronic Structures of Transition Metal Atoms and Ions
In the Periodic Table, there are three transition metal series: 1st, Sc-Zn; 2nd, Y-Cd; and 3rd, La-Hg. The
chemical and physical properties of the transition metal atoms and elements do not change periodically across a
period as much as the representative atoms and elements of the same period. Thus, there are similarities in
properties not only in families (columns) but also across periods for the transition metal atoms and elements.
The transition metal atoms have a great variety of oxidation states. For example, manganese can form oxidation
states of +2, +3, +4, +5, +6, and +7. Thus, the transition metals can form a great variety of ionic and covalent
Many transition metal complexes are colored. The color in these species is due to electronic transitions
involving d-electrons, and the energies of these transitions are related to the ligands, the oxidation state of the
transition metal atom, and the n value in ns2(n-1)dx.
A common metal complex is Ag(NH3)2+, formed when Ag+ ions are mixed with neutral ammonia molecules.
Ag+ + 2 NH3 -> Ag(NH3)2+
A complex Ag(S2O3)23- is formed between silver ions and negative thiosulfate ions:
Ag+ + 2 S2O32- -> Ag(S2O3)23-
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
CN = 4
ligands attached at the corners of a tetrahedron
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
CN = 2
ligands and metal atom connected in a straight line
CN = 6
ligands attached at the corners of an octahedron
Linkage isomers are two or more coordination compounds in which the donor atom of at least one of the ligands
is different (i.e., the connectivity between atoms is different).
This type of isomerism can only exist when the compound contains a ligand that can bond to the metal atom in
two (or more) different ways. Some ligands that can form linkage isomers are shown below.
Not all coordination compounds have linkage isomers.
Linkage isomers have different physical and chemical properties.
Ligands That Can Form Linkage Isomers
ligand Lewis structure name donor atoms
CN- cyanide ion C or N
SCN- thiocyanate ion S or N
NO2- nitrite ion N or O
Structures With Bidentate Ligands
Bidentate ligands are Lewis bases that donate two pairs ("bi") of electrons to a metal atom.
Bidentate ligands are often referred to as chelating ligands ("chelate" is derived from the Greek word for
"claw") because they can "grab" a metal atom in two places.
A complex that contains a chelating ligand is called a chelate.
Some Bidentate Ligands
ethylenediammine acetylacetonate ion
phenanthroline oxalate ion
Complex ions usually refer to cations in which surrounding water molecules have been replaced by some other
electron pair donor. We write:
Cu2+ --> Cu(NH3)42+
In this reaction, we neglect water molecules surrounding the copper(II) ion. Complex ions are studied for many
reasons, not the least of which is that many such ions are very colorful:
The well at the left contains copper chloride and water. Beginning from the right, the wells contain increasing
amounts of aqueous ammonia solution. The deep blue color in the 2nd well from the left is due to Cu(NH3)42+.
The pale blue color at the left is due to Cu(H2O)42+ and some copper chloride complexes (greenish).
Questions that deal with the interplay of precipitation reactions and complex ion formation frequently are found
on the AP exam. Here is a sequence of reactions of Ag+:
These reactions are written as:
Ag+ + CO32- --> Ag2CO3
Ag2CO3 + PO43- --> Ag3PO4 + CO32-
Ag3PO4 + OH- --> Ag2O + PO43- + H2O
Ag2O + Cl- + H+ --> AgCl + H2O
AgCl + NH3 --> [Ag(NH3)2]+ + Cl-
Ag(NH3)2+ + Br- --> AgBr + NH3
AgBr + S2O32- --> [Ag(S2O3)2]3- + Br-
[Ag(S2O3)2]3- + I- --> AgI + S2O32-
AgI + HS- --> Ag2S + I- + H+
The silver ammine complex, Ag(NH3)2+, is a relatively frequent visitor on AP tests. Many cations (Ag+, Cu2+,
Cd2+, Zn2+, and many others) form complexes with ammonia. These sometimes are called ammine complexes.
Another reaction type frequent seen is the formation of a hydroxide followed by the formation of a soluble
hydroxide complex ion. Fe(OH)3 and Mg(OH)2 do not form soluble complexes, but aluminum and zinc do:
Zn(OH)2 + OH- --> Zn(OH)42-
Al(OH)3 + OH- --> Al(OH)4-
Finally, a favorite reaction is that of ferric ion with thiocyanante ion (SCN-):
Fe3+ + SCN- FeSCN2+
Complex ions are soluble in solutions because they are charged. The forces of attraction between the complex
ions and water molecules are quite large. (ion-dipole intermolecular forces)
In this course, you will encounter complex ions of ammonia and hydroxide.
Cations that form complex ions with excess NH3 are:
Cations that form complex ions with excess OH- are:
The formation of a complex ion is suspected when:
1. The addition of a reagent causes precipitate formation followed by dissolving of the precipitate on
addition of excess reagent.
2. The formation of a colored species in solution is observed.
Now you try some!
A. Can you give the molecular formulas of the following coordination compounds?
1. hexaammineiron(III) nitrate ___________________________________
2. ammonium tetrachlorocuprate(II) ___________________________________
3. sodium monochloropentacyanoferrate(III) ___________________________________
4. potassium hexafluorocobaltate(III) ___________________________________
B. Can you give the name of the following coordination compounds?
5. [CoBr(NH3)5]SO4 ____________________________________________________________________
6. [Fe(NH3)6][Cr(CN)6] ______________________________________________
7. [Co(SO4)(NH3)5]+ _____________________________________________________________________
8. [Fe(OH)(H2O)5]2+ _____________________________________________________________________
C. Draw the square planar arrangements for the cis/trans isomers of [Pt(NH3)2Cl2]:
D. Draw cis/trans isomers of Cr(H2O)4Cl2]+ and [Cr(NH3)4Cl2]+ coordination number = ____
E. The complex ion [Co(NH3)5(H2O)]3+ will react in a solution of sodium nitrite (NaNO2) and nitrous acid
(HNO2) to substitute a nitrite anion for the water ligand. This may sound like a simple ligand switch, but
the mechanism of the reaction is more complex.
The nitrite ion, NO2-, has two different resonance structures, each with the negative charge on a different
These two linkage isomers of the NO2- ligand introduce a complexity to the mechanism governed by kinetics,
experiments will distinguish between two possible mechanisms for the ligand substitution reaction:
aqua nitrito nitro (1)
aqua nitro nitrito (2)
F. Draw the structural formulas for these two linkage isomers.
G. Write net ionic equations for the following:
a) A drop of potassium thiocyanate is added to a solution of iron(III) chloride.
b) A concentrated solution of ammonia is added to a suspension of zinc hydroxide.
c) Excess concentrated potassium hydroxide solution is added to a precipitate of zinc hydroxide.
d) Excess concentrated sodium hydroxide solution is added to solid aluminum hydroxide.
e) A concentrated solution of ammonia is added to a solution of copper(II) chloride.
f) A solution of ammonium thiocyanate is added to a solution of iron(III) chloride.
g) A Excess sodium cyanide is added to a solution of silver nitrate.
h) Excess concentrated ammonia solution is added to a solution of nickel(II) sulfate.