Coordination compounds
Complexes
• A central metal atom bonded to a group
of molecules or ions is a metal complex.
• The ligands donate electrons to the metal
via coordinate covalent bonds.
• If it’s charged, it’s a complex ion.
• Compounds containing complexes are
coordination compounds.
• Typically consists of a complex ion and counterions
(anions or cations as needed to produce a neutral
compound):
[K3Fe(CN)6], [Co(NH3)5Cl]Cl2
Lewis acids and bases
A Lewis base is a molecule or ion that donates a
lone pair of electrons to make a bond
NH3 - -
Examples: OH2 Cl F
A Lewis acid is a molecule of ion that accepts
a lone pair of electrons to make a bond
+ 3+ 2+ n+
Examples: H Co Co M
Coordination Chemistry
• Transition metals act as Lewis acids
Form complexes/complex ions
Fe3+(aq) + 6CN-(aq) [Fe(CN)6]3-(aq)
Lewis acid Lewis base Complex ion
Ni2+(aq) + 6NH3(aq) [Ni(NH3)6]2+(aq)
Lewis acid Lewis base Complex ion
Complex with a net charge = complex ion
Coordinate Covalent Bond
Bond resulting from the interaction between a Lewis
base (the ligand) and a Lewis acid (the metal ion).
Werner’s Theory
Co(III) oxidation state
Coordination is 6
Coordination chemistry emerged from the
work of Alfred Werner, a Swiss chemist
who examined different compounds
composed of cobalt(III) chloride and
ammonia.
• suggested in 1893 that metal ions have primary and
secondary valences.
– Primary valence - the oxidation state
– Secondary valence - the coordinate number
Coordination Chemistry
• Coordination sphere
– Metal and ligands bound to it
• Coordination number
Number of bonds formed between the metal ion and the
ligands in the complex ion.
6 and 4 (most common)
2 and 8 (least common)
Werner’s explanation of coordination complexes
Metal ions exhibit two kinds of valence: primary and
secondary valences
The primary valence is the oxidation number (positive
charge) of the metal (usually 1+, 2+ or 3+)
The secondary valence is the number of atoms that are
directly bonded (coordinated) to the metal
The secondary valence is also termed the “coordination
number” of the metal in a coordination complex
Werner’s Theory
• The central metal and the ligands directly bonded to it
make up the coordination sphere of the complex.
• In CoCl3 ∙ 6 NH3, all six of the ligands are NH3 and the 3
chloride ions are outside the coordination sphere.
Werner’s Theory
In CoCl3 ∙ 5 NH3 the five NH3 groups and one
chlorine are bonded to the cobalt, and the other
two chloride ions are outside the sphere.
Werner’s Theory
Werner proposed putting all molecules and ions
within the sphere in brackets and those “free”
anions (that dissociate from the complex ion when
dissolved in water) outside the brackets.
Werner’s Theory
Two forms of CoCl3 ∙ 4 NH3.
– The formula would be written
[Co(NH3)4Cl2]Cl.
– One of the two forms has the two
chlorines next to each other.
– The other has the chlorines
opposite each other.
Coordination Chemistry
Complex charge = sum of charges
on the metal and the ligands
[Fe(CN)6]3- Fe3+
CN-
+3 6(-1)
Coordination Chemistry
Neutral charge of coordination
compound = sum of charges on
metal, ligands, and
counterbalancing ions
[Co(NH3)6]Cl2
+2 6(0) 2(-1)
neutral compounds
Coordination Chemistry
• Ligands
– classified according to the number of
donor atoms
– Examples
• monodentate = 1
• bidentate = 2 chelating
• tetradentate = 4 agents
• hexadentate = 6
• polydentate = 2 or more donor
atoms
Ligands
• Monodentate
– Examples:
• H2O, CN-, NH3, NO2-, OH-,
X- (halides), CO, O2-
–Example Complexes
• [Co(NH3)6]3+
• [Fe(CN)6]3-
The Bidentate
Ligand
Ethylenediamine
and the
Monodentate
Ligand Ammonia
Bidentate Ligands
O O 2- CH2 CH2
H2N NH2
C C
*Ethylenediamine*(en)
O O
* oxalate ion
*
CH
– Example Complexes *N CH
• [Co(en)3]3+
*N C CH
HC C C
• [Cr(C2O4)3]3- HC C CH
CH CH
• [Fe(NH3)4(o-phen)]3+
ortho-phenanthroline
Ligands
H C
C
O
M M N
oxalate ion Ethylenediamine (en)
Ligands
NH2CH2CH2NH2
Ethylene glycol
H2C OH H
H2C O O CH2
Cu
H2C OH H2C O O CH2
H
Ligands
• Hexadentate
– ethylenediaminetetracetate
– (EDTA) =
(O2CCH2)2N(CH2)2N(CH2CO2)24-
– Example Complexes
• [Fe(EDTA)]-1
• [Co(EDTA)]-1
Ligands
O EDTA O
*O C CH 2 CH 2 C O*
*
N *
CH 2 CH 2 N
*O C CH 2 CH 2 C O*
O O
Ligands
EDTA
Factors Affecting Coordination
Number
1. The size of the central atom or ion.
2. Steric interactions between ligands
3. The electronic structure of the metal
atom or ion. If the oxidation number is
high, the metal can accept more electrons
from the (Lewis base) ligands. Metals with
many d electrons will have lower
coordination numbers.
Alfred Werner determined the formulas and
structures of many transition metal compounds
by studying their isomers. Due to the existence
of a variety of structural isomers, he proposed
that complexes must have square planar,
tetrahedral and octahedral shapes.
Octahedral:
Most important Square planar
Tetrahedral
Common Geometries of Complexes
Coordination Number Geometry
Linear
Example: [Ag(NH3)2]+
Common Geometries of Complexes
Coordination Number Geometry
4
tetrahedral
Examples: [Zn(NH3)4]2+,
[FeCl4]-
square
planar
Example: [Ni(CN)4]2-
Common Geometries of Complexes
Coordination Number Geometry
6
Examples: [Co(CN)6]3-,
[Fe(en)3]3+
octahedral
Possible Structures for 6-Coordinate Cobalt
Possible Structures for 6-Coordinate Cobalt
Some Classes of Isomers
Structural Isomerism
• Coordination Isomerism:
Composition of the complex ion varies.
[Cr(NH3)5SO4]Br and [Cr(NH3)5Br]SO4
• Linkage Isomerism:
Composition of the complex ion is the
same, but the point of attachment of at least
one of the ligands differs.
Structural Isomers
Linkage Isomerism of NO2–
If a ligand (like the NO2
group at the bottom of the
complex) can bind to the
metal with one or another
atom as the donor atom,
linkage isomers are
formed.
Linkage isomers
Example: S C N Bonding to metal may occur at
the S or the N atom
Bonding occurs from
N atom to metal
Bonding occurs from
S atom to metal
Stereoisomerism
• Geometrical Isomerism (cis-trans):
Atoms or groups of atoms can assume
different positions around a rigid ring or
bond.
Cis – same side (next to each other)
Trans – opposite sides (across from each
other)
Geometrical (cis-trans) Isomerism for a
Square Planar Compound
(a) cis isomer (b) trans isomer
Diaminodichloroplatinum
PtCl2.2NH3 ; [ Pt(NH3)2Cl2]
Cis isomer Тtrans isomer
Cl NH3 Cl NH3
Pt Pt
Cl NH3 NH3 Cl
Orange color Yellow color
cisplatin
Antiumor medicament produces deafness
GeometricIsomers
cis trans
isomer isomer
[Co(H2O)4Cl2]+
Geometrical (cis-trans) Isomerism for an
Octahedral Complex Ion
Some critical enzymes in our cells are metalloproteins,
giant biolmolecules which contain a metal atom
These metalloproteins control key life processes such as
respiration and protect cells against disease
Vitamin B12, which prevents pernicious anemia, contains a
Co atom which gives the vitamin a red color
1964
Vitamin B12 (Co[C62H88N13O14P])CN
Involved in many important biological processes, including
the production of red blood cells
A very important porphine that converts solar
photons into food energy: chlorophyll
Chlorophyll (C55H72N4O5Mg)
Biological Importance of Iron
• Plays a central role in almost all living cells.
• Component of hemoglobin and myoglobin.
• Involved in the electron-transport chain.
The Heme Complex
These planar molecules have a
“hole” in the center which to
which a metal can coordinate
NH NH
N
Hemoglobin is a metalloprotein which contains an iron atom
and transports O2 through out living systems
Reversible addition of O2 to hemoglobin
The mechanism by which oxygen is carried throughout
the body
Myoglobin
Myoglobin, a protein that
stores O2 in cells
• The Fe2+ ion is coordinated to
four nitrogen atoms in the
porphyrin of the heme (the
disk in the figure) and on
nitrogen from the protein
chain.
• This leaves a 6th coordination
position available for an
oxygen molecule.
END
