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Chapter 22: Co-ordination Complexes
Ligands and Co-ordination Complexes
The first experiment in the Chemistry 2000 lab is synthesis and
analysis of a complex salt. The name of this complex salt is
potassium trisoxalatoferrate(III) trihydrate. Its formula is
K3[Fe(C2O4)3]·3H2O and its structure is shown below:
O
C
O
O
K3
O
C
O
C
O
O
C
O
+
Fe
O
O
C
C
3 H2O
O
O
Co-ordination complexes are compounds in which several
ligands are co-ordinated to a transition metal cation. A ligand is
any substance (neutral or anionic) which can act as a Lewis
base, donating electrons to the transition metal (which acts as a
Lewis acid).
e.g. Cu(NH3)42+ is Cu2+ with four :NH3 ligands
e.g. Zn(CN)42- is Zn2+ with four :CN- ligands
Note that the ligands do not have to be the same!
Ligands co-ordinated to a transition metal though one atom are
called monodentate ligands. Those co-ordinated to a transition
metal through two atoms (like in the picture above) are called
bidentate (“two-toothed”) ligands. Polydentate ligands can also
be called chelating ligands, or chelates (“claws”).
O
We saw one such ligand in the Chemistry
1000 Water Hardness lab. EDTA was able
to “grip” a cation by co-ordinating to it with
six different atoms! (For clarity, individual
carbon atoms are not shown.)
O
N
O
M
O
O
N
O
O
The number of atoms attached to the transition metal is referred
to as the co-ordination number. It doesn’t matter whether
these atoms come from the same molecule/ion or from several
different ones.
e.g. The co-ordination number is 6 for both Fe(C2O4)33- and
Fe(OH2)63+
Complex Salts
As shown on the previous page, co-ordination complexes can
have positive charges, negative charges or be neutral. To make
a neutral salt, charged co-ordination complexes (also called
complex ions) will need one or more counter ions to balance the
charge. This gives a complex salt.
e.g. In K3[Fe(C2O4)3]·3H2O, the negative charge of Fe(C2O4)33is balanced out by the three K+ cations.
O
Some co-ordination complexes and complex salts contain extra
water molecules which were trapped during crystallization.
These complexes are also hydrates. (Recall Chemistry 1000!)
Thus, a co-ordination complex must contain a transition metal
cation and several ligands. It may also have counter ion(s) (to
balance charge) or extra water molecules. When naming a coordination complex or complex salt, look for these components.
Naming Complex Salts
The first step in naming a complex salt is to identify the
complex ion. To name the complex ion:
1. List the ligands using prefixes to indicate the number of each
type of ligand.
• For ligands with simple names (e.g. chloro, hydroxo), use
di, tri, tetra, etc.
• For ligands with complicated names (e.g. oxalato), use bis,
tris, and tetrakis.
2. Name the transition metal. If the complex ion is an anion, use
the metal’s Latin name and change the suffix to ‘ate’
3. List the oxidation state using Roman numerals.
Once you have named the complex ion, name the complex salt
like any other ionic compound: cation then anion then hydration.
e.g. Fe(C2O4)33- = 3 C2O42+
Fe3+
K3[Fe(C2O4)3]·3H2O
potassium trisoxalatoferrate(III) trihydrate
(cation)
(complex anion) (hydration)
Table of Common Ligands
Anions
Formula
fluoride anion
:Fchloride anion
:Clnitrite anion
:NO2:ONOcarbonate anion
:OCO222OO
oxalate anion
[
cyanide anion
thiocyanate anion
:OCCO:
:CN:SCN:NCS:H:O2:OH-
hydride anion
oxide anion
hydroxide anion
Neutral Molecules
water
:OH2
ammonia
:NH3
carbon monoxide
:CO
nitrogen monoxide
:NO
Name the following complex salts
(a) [Ni(OH2)6]CO3
(b) [Cu(NH3)4]SO4 · H2O
]
Name
fluoro
chloro
nitro
nitrito
carbonato
oxalato
cyano
thiocyano
isothiocyano
hydrido
oxido
hydroxo
aqua
ammine
carbonyl
nitrosyl
Note that there is a difference between water as a ligand and
“water of crystallization”. The bright blue crystals commonly
referred to as CuSO4·5H2O are really [Cu(OH2)4]SO4·H2O. Give
the name corresponding to each of these two formulas.
CuSO4·5H2O =
[Cu(OH2)4]SO4·H2O =
The only way to determine this information is by experiment,
but you should recognize that, in many hydrated salts, at least
some of the water molecules serve as ligands.
Why Are Transition Metals Special?
As we saw in Chemistry 1000, metals in Groups 1 and 2 are
limited in what oxidation states they can take on. Transition
metals, on the other hand, can take on many different oxidation
states. This distribution is not entirely random, as show in the
graph below (with common oxidation states in dark red):
The elements in the middle can exist in a wider variety of
oxidation states than those on either end of the d-block. Why?
Because of the valence d electrons! Compared to s and p
electrons, d electrons can be added or removed relatively easily.
e.g. The electron configuration of neutral vanadium is:
The first two electrons removed will be those in the 4s
orbital. After that, electrons are removed from the 3d
orbitals giving three stable oxidation states:
vanadium(III)
vanadium(IV)
vanadium(V)
Electronic Structure and Colour
One of the more fun consequences of these partially filled d
subshells is that the co-ordination complexes of transition metals
are often brightly coloured. The flasks below contain aqueous
solutions of several nitrate salts. Note that, since all nitrates are
water-soluble, these solutions contain transition metal-water
complexes.
Fe3+
Co2+
Ni2+
Cu2+
Why is the Zn2+ complex the only colourless one?
Zn2+
Consider the electron configurations of the five cations:
Fe3+
Co2+
Ni2+
Cu2+
Zn2+
The colourless Zn2+solution is the only one shown containing a
cation with a full d subshell!
Where does the variety in colour come from?
Most co-ordination complexes have octahedral geometry. This
means that two of the d orbitals point directly at ligands while
the other three do not.
point at ligands
point between ligands
A simple electrostatic model, called the crystal field theory,
assumes that there will be a certain degree of electron-electron
repulsion between the electron pair a ligand donates and any
electrons already in the metal d orbitals. This repulsion is felt
most strongly by electrons in d orbitals pointing at the ligands.
Thus, the dz2 and dx2-y2 orbitals are pushed to higher energy than
the dxy, dxz and dyz orbitals. This separation in energy is referred
to as crystal field splitting (∆o where ‘o’ is for ‘octahedral’).
How does this make for coloured solutions?
Recall that photons are emitted when electrons drop from a
higher energy orbital to a lower energy orbital. (see Atomic Line
Spectra in Chemistry 1000) Similarly, the electrons get to the
higher energy orbital by absorbing photons of light.
In co-ordination complexes with crystal field splitting, there are
two ways to distribute d electrons. The high spin distribution
maximizes the spin pairing of the d electrons while the low spin
distribution puts electrons in the lowest energy orbitals first.
When the atom (of the ligand) donating the electron pair is
oxygen, the d electrons are always distributed high spin.
Electrons in the lower energy d orbitals can absorb photons and
be excited into the higher energy d orbitals. Since ∆o
corresponds to the energy of light in the visible region (and there
is more than one way to absorb a photon), some wavelengths of
visible light are absorbed. The wavelengths that are not
absorbed give the colour of solution.
e.g. Ni2+ in Ni(OH2)62+ absorbs both red-yellow and violet light,
giving a solution that appears green:
Note that different ligands provide different amounts of crystal
field splitting. Fe(OH2)63+ and Fe(C2O4)33- are both complexes
of Fe3+ but Fe(OH2)63+ is red-orange while Fe(C2O4)33- is green.
A spectrophotometer measures the amount of light absorbed by
a complex. When analyzing a green complex, it is therefore
necessary to look at the absorption of light other than green.
Generally, we will try to choose the wavelengths most strongly
absorbed by the complex. On the figure above, that would
correspond to the peak yellow-red wavelength or the peak violet
wavelength.
Important Concepts from Chapter 22
• Lewis acids and Lewis bases
• ligands (monodentate, bidentate, chelating)
• co-ordination number
• naming co-ordination complexes and complex salts
• electron configurations of cations
• d electrons and crystal field splitting
• why co-ordination complexes are often coloured
• spectrophotometry (especially for labs!)
Appendix to Chapter 22
Latin Names of Some Common Transition Metals
Element Symbol
Latin Name
Name in Anionic
Name
Complex
chromium
Cr
chromum
chromate
manganese
Mn
manganum
manganate
zinc
Zn
zincum
zincate
copper
gold
iron
mercury
Cu
Au
Fe
Hg
silver
Ag
cuprum
aurum
ferrum
hydrargyrum
literally, “liquid silver”
argentum
cuprate
aurate
ferrate
hydrargyrate
argentate