Chapter 25 : Complex Ions
and Coordination Compounds
25-1 Werner’s Theory of Coordination Compounds: An Overview
25-5 Bonding in Complex Ions: Crystal Field Theory
25-6 Color and the Colors of Complexes
25-7 Aspects of Complex Ion Equilibria
25-8 Acid-Base Reactions of Complex Ions
25-9 Some Kinetic Considerations
25-10 Applications of Coordination Chemistry
25-1 Werner’s Theory of Coordination Compounds:
Complex Ion - is a polyatomic cation or anion composed
of a central metal ion to w/c are bonded other groups
(molecules or ions)
Substances containing complex ions belong to a category of
compounds called coordination compounds
The following formulas describe a series of three
CoCl3.6NH3 CoCl3.5NH3 CoCl3.4NH3
(a) (b) (c)
In 1893, Swiss chemist Alfred Werner proposed that certain
metal atoms, primarily those of the transition metals, have two
types of valence.
1. The primary valence, is based on the number of electrons
the atom loses in forming the metal ion.
2. Secondary or auxiliary, valence is responsible for the
bonding of other groups, called ligands, to the
central metal ion.
[Co(NH3)6]Cl3 [CoCl(NH3)5]Cl2 [CoCl2(NH3)4]Cl
(a) (b) (c)
Ionization of coordination compound (a) can be represented as
[Co(NH3)6]Cl3(s) [Co(NH3)6]3+(aq) + 3 Cl-(aq)
In the presence of an excess of AgNO3(aq)
(a) yield three moles of AgCl(s) per mole of compound
(b) yield only two moles of AgCl(s) per mole of compound
(c) and yield only one moles of AgCl(s) per mole of compound
Werner’s theory also accounts for
1. The coordination compound [CoCl3(NH3)3], w/c is a
nonelectrolyte and yields no precipitate with AgNO3(aq).
2. The compound Na[CoCl4(NH3)2], in w/c the complex
ion has a net negative charge, that is [CoCl4(NH3)2]-
are all complex ions, and because they
[CoCl(NH3)5]2+ carry a positive charge, they are
[CoCl4(NH3)2]- is also a complex ion, but it is an anion.
[CoCl3(NH3)3], is a neutral molecule.
- is the number of points around the metal center at w/c bonds
to ligands can form.
- ranging from 2 to 12 have been observed in complexes,
although the number 6 is by far the most common,
follow by 4.
- 2 is most limited to complexes of Cu(I), Ag(I), and Au(I).
- greater than 6 are not often found in members of the first
transition series but are more common in those of the
second and third series.
- stable complexes w/ coordination no. 3 and 5 are rarely
- observed in a complex depends on a number of factors, such as
the ratio of the radius of the central metal atom or ion to those of
the attached ligands.
Table 25.1 Some Common Coordination
Number of Metal Ions
Cu+ 2, 4
Au+ 2, 4 Al3+ 4, 6
Fe2+ 6 Fe3+ 6
Co2+ 4, 6 Co3+ 6
Ni2+ 4, 6 Au3+ 4
Cu2+ 4, 6
Figure 25-1 Structure of complex ions. Attachment of the NH3
molecules occurs through the lone pair electrons on the N atoms.
Relating the Formula of a Complex to the Coordination
Number and Oxidation State of the Central Metal. What are
the coordination number and oxidation state of Co in the
complex ion [CoCl(NO2)(NH3)4]+?
charge of 1- on Cl - total
oxidation state = x negative
charge of 1- on NO2 - charge: 2-
net charge on complex ion
x - 2 = +1
coordination number = 6 x = +3
Practice Example A: What are the coordination number and
oxidation state of nickel in the ion [Ni(CN)4I]3 - ?
Ans. Coordination Number is
5 Oxidation State is +2
Practice Example B: Write the formula of a complex with
cyanide ion ligands, an iron ion with an oxidation state of
+3, and a coordination number of 6.
Ans. [Fe(CN)6]3 -
A common feature shared by the ligands in complexes is the
• ability to donate electrons pairs to central metal atoms
or ions act as Lewis bases.
• ability in accepting electrons pairs, central atoms
or ions act as Lewis acids.
A ligand that uses one pair of electrons to form one point of
attachment to the central metal atom or ion is called
Some ligands are capable of donating more than a single electron
pair from different atoms in the ligand and to different sites in the
geometry structure of a complex are called polydentate ligands.
Table 25.2 Some Common Monodentate Ligands
Formula Name as Ligand Formula Name as Ligand
H2O aqua F- fluoro
NH3 ammine Cl - chloro
CO carbonyl Br - bromo
NO nitrosyl I- iodo
CH3NH2 methylamine O 2- oxo
C5H5N pyridine OH -
hydroxo CN -
nitroa ONO -
nitritoa SCN -
thiocyanatob NCS -
The molecule ethylenediamine (en) can donate two electron
pairs, one from each N atom.
Bidentate ligand attaches itself to the central atom of a complex
at two points in the coordination sphere.
Table 25.3 Some Common Polydentate Ligands
Abbreviation Name Formula
Figure 25-2 Two representations of the chelate [Pt(en)2]2+.
The ligands attach at adjacent corners along an edge of the
square. They do not bridge the square by attaching to opposite
corners. Bonds are shown in red, and the square planar shape is
indicated by the black parallelogram.
Chelate results from the attachment of polydentate ligands to the
central atom of a complex ion.
Chelating agent is a polydentate ligand. It simultaneously
attaches to two or , more position in the coordination sphere of
the central atom of a complex ion.
1. In names and formulas of coordination compounds,
cations come first, followed by anions.
2. Anions as ligands are named by using the ending “o”.
Normally, “ide” ending change to “o,” “ite” to
“ito,” and “ate” to “ato”
3. Neutral molecules as ligands generally carry the unmodified
name. For ex., the name ethylenediamine is used both for the
free molecule and for the molecule as a ligand. Aqua, ammine,
carbonyl, and nitrosyl are important exceptions.
4. The number of ligands of a given type is denoted by a prefix.
- the usual prefixes are mono = 1, di = 2, tri = 3, tetra = 4 and
- if the ligand name is a composite name that itself contains a prefix,
such as ethylenediamine, place parentheses around the
name and precede it w/ bis = 2, tris = 3, tetrakis = 4, and so on...
5. In naming a complex, ligands are named first, in
alphabetical order, followed by the name of the metal center.
The oxidation state of the metal center is donated by a
Roman numeral. If the complex is an anion, the ending “ate”
is attached to the name of the metal.
- prefixes (di,tri,bis,tris,…) are ignored in establishing the
[CrCl2(H2O)4]+ is called tetraaquadichlorochromium(III) ion
[CoCl2(en)2]+ is dichlorobis(ethylenediamine)cobalt(III) ion
[Cr(OH)4]- is tetrahydroxochromate(III) ion
6. In writing the formula of a complex, the chemical symbol
of the metal center is written first, followed by the
formulas of anions and then neutral molecules.
- if there are two or more different anions or neutral
molecules as ligands, they are written in alphabetical order
according to the first chemical symbols of their formulas.
the formula of the tetraaminechloronitrocobalt(III) ion,
Cl- precedes NO2-, and both are placed ahead of the
neutral NH3 molecules:
Relating Names and Formulas of Complexes. (a) What is the
name of the complex [CoCl3(NH3)3]? (b) What is the formula of
the compound pentaaquachlorochromium(III) chloride?
(c) What is the name of the compound K3[Fe(CN)6]?
(a) [CoCl3(NH3)3] consist of three ammonia molecules and three
chloride ions attached to a central Co3+ ion; it is electrically
neutral. The name of this neutral complex is
(b) The central metal ion is Cr3+. There are five H2O molecules
and one Cl - ion as ligands. The complex ion carries a net charge
of 2+. Two Cl - ions are required to neutralize the charge on this
complex cation. The formula of the coordination compound is
(c) This compound consist of K+ cation and complex anions
having the formula [Fe(CN)6]3 -. Each cyanide ion carries a
charge of 1-, so the oxidation state the iron must be +3. The
Latin-based name “ferrate” is used because the complex ion is
an anion. The name of the anion is hexacyanoferrate(III) ion.
The coordination compound is
Practice Example A: What is the formula of the compound
Practice Example A: What is the name of the compound
Ans. Pentaamminethiocyanatocobalt(III) chloride
Isomers are substances that have the same formulas but
differ in their structures and in their properties.
Kinds of isomerism can be lumped into two broad categories:
Structural isomers have the same number and kinds of atoms
but they differ in their structural formulas.
Isomerism, Coordination Isomerism, & Linkage
Stereoisomers the number and types of atoms and bonds in
molecules are the same, but certain atoms are oriented
differently in space.
Geometric Isomerism, and Optical Isomerism
Some ligands may attach to the central metal ion of a
complex ion in different ways.
tetraaminechloronitrocobalt(III) ion (a)
tetraaminechloronitritocobalt(III) ion (b)
• in organic compounds refers to the existence of
nonequivalent structures (cis and trans) that differ in the
positioning of substituent groups relative to a double bond.
• in complexes the nonequivalent structures are based
on the positions at which ligands are attached to the
(a) No isomerism
Substitute third Cl -
Substitute third Cl - (b) Cis and Trans isomerism
(c) No isomerism
- also called enantiomers(nonsuperimposable mirror
images), are isomers that differ only in the way they rotate
the plane of polarized light.
• Structures that are nonsuperimposable mirror images of each
other are called enantiomers and are said to be chiral.
• Structures that are superimposable are achiral.
Enantiomer rotates the plane of polarized light to the right
(clockwise) is said to be dextrorotatory (designated + or d).
Enantiomer rotates the plane of polarized light to the left
(clockwise) is said to be levorotatory (designated - or l).
25-5 Bonding in Complex Ions: Crystal
Crystal field theory describes bonding in complex ions
in terms of electrostatic attractions between ligands and
the nucleus of the central metal ion.
- particular attention is focused on the splitting of the d
energy level of the central metal ion that results from
Different ligands can be arranged in order of their abilities to
produce a splitting of the d energy levels, this arrangement is
known as the spectrochemical series.
The d-orbital energy levels of the free central ion are raised in
the presence of ligands to the average level shown, but the five
levels are split into two groups.
d d x2
x2 - y2
Average energy of
d orbitals in field
dxy dxz dyz
Figure 25-3 Splitting of d energy levels in the formation of an
octahedral complex ion.
Thank you for