substances
JOSE IGNACIO SOTO/FOTOlIA
Iridium
Life-saving transition
element
So many colours
Exam lINks
The terms in bold link to topics in the AQA, Edexcel, OCR, WJEC
and CCEA A-level specifications, as well as the Highers/Advanced
Highers exam specifications.
Iridium is a transition metal that gives rise to coloured species
due to the electron configuration of the metal. Phosphorescent
complexes of iridium (and other related metals) with organic
ligands can be used as sensors for pH changes and oxidation
state.
I
ridium (Ir) is a little-known element in the third row of
the transition metal block of elements (Figure 1). Its atomic
number is 77 and it has a ground state electron configuration
of [Xe] 4f14 5d7 6s2.
In this region of the periodic table are ‘jewellery metals’ such
as silver (Ag) and gold (Au), extremely dense and unreactive
metals like platinum (Pt) and osmium (Os), and metals with
very low abundance in the Earth’s crust such as ruthenium (Ru)
and rhodium (Rh).
Iridium fits the pattern well, as it is a very hard, brittle,
silvery-white metal with a density almost three times that of
iron. It shows the greatest degree of resistance to chemical
attack of any metal element. Only about 3 tonnes of iridium are
produced and used across the world per year.
Iridium was discovered, along with osmium, by Smithson
Tennant and William Wollaston in 1803 among insoluble
impurities in natural platinum. Tennant, the primary discoverer,
was born in Selby, near York on 30 November 1761. This article
marks the 250th anniversary of his birth (Box 1). Thanks to the
classical influence of his father, Tennant named iridium after
the goddess Iris, personification of the rainbow, because of the
striking colour of many of its compounds.
Transition metals are defined as those metals having partly
filled d orbitals. In aqueous solution, these ions are hydrated
by six water molecules, referred to as ligands, in an octahedral
array (Figure 2). These ligands are linked to the metal ion by
dative covalent bonds with d orbital electrons from the metal.
When the ligands bond with the transition metal ion, there is
repulsion between the electrons in the ligands and the electrons
in the d orbitals of the metal ion. This raises the energy of the
d orbitals.
As the d orbitals are not symmetrically arranged (Figure 3),
they are not equally repelled by the approach of the electrons
from the ligands. Instead of five orbitals with equal energy, the
three orbitals on the top row of Figure 3 have a slightly lower
energy than the other two.
The coloured complexes that we are interested in contain
Ir in the oxidation state of +3, Ir(iii), where the atom has
18
1
H
2
Li Be
Na Mg
4
5
K Ca Sc Ti
V
Rb Sr
3
Y
6
7
8
9
10
11
12
He
13
14
15
16
17
B
C
N
O
F Ne
Al
Si
P
S
Cl Ar
Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te
Cs Ba La Hf Ta W Re Os
Ir
I
Xe
Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg Cp Uut Uuq Uup Uuh Uus Uuo
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
Figure 1 Iridium is one of the transition metal elements
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Figure 2 Generalised
diagram of an octahedral
arrangement of ligands
(in grey) around a central
metal atom (in pink)
Chemistry Review
29/09/2011 10:53
Box 1
Smithson Tennant (1761–1815)
Red
Tennant was the only child of the marriage between a clergyman
and the daughter of the local apothecary. He showed an early
interest in practical science, as it is said he made gunpowder for
his own experiments by the age of 9 and attended talks by visiting
lecturers on ‘natural philosophy’, as experimental science was then
referred to. His headmaster at Beverley Grammar School encouraged
his students to read widely: Tennant took in, among other titles,
Newton’s Optics. He is alleged to have been inspired to focus
moonbeams to melt butter following this reading — the results of
this experiment are not recorded.
Magenta
Yellow
Blue
Green
Cyan
Figure 4 A simple colour wheel
The well-known blue colour of Cu 2+ is caused by the
absorption of light from the yellow area of the spectrum. Blue
is the complimentary colour to yellow, so the Cu 2+ salt appears
blue. Complimentary colours are those opposite each other on
the colour wheel (Figure 4). Ir(iii) complexes absorb in the cyan
region, so produce a vivid red colour.
The variety of colours found in iridium salts results from the
many oxidation states it shows. There are nine in all, from –3 to
+6. For instance, while Ir(iii) is commonly red, Ir(iv) is brown
and Ir(vi) is yellow.
Phosphorescence
lost 3 electrons. Since iridium’s ground state is [Xe] 4f14 5d7 6s2,
the Ir(iii) species has the electron structure of [Xe] 4f14 5d6. As
each orbital can contain 2 electrons, all the 5d6 electrons are
contained in the three d orbitals of lower energy.
If the Ir(iii) ion is exposed to energy of an appropriate
wavelength, in this case normal white light, one of these d
electrons can be ‘excited’ into one of the higher energy orbitals.
This is an excited state. The amount of energy needed for this
promotion depends on both the metal and the ligands. If
different amounts of energy are absorbed, different colours
are seen.
z
z
z
y
x
z
dyz
y
x
z
dxz
y
x
dxy
More complex ligands with aromatic ring systems have further
absorption bands due to delocalised electrons in the ring
systems, and interaction between these systems and the
d orbitals. These organometallic complexes have many excited
states, with many possible ways that energy can be absorbed and
released. For iridium the relevant process is phosphorescence.
Here, energy is absorbed rapidly, but released relatively slowly
as light. Common examples of phosphorescence are glow-inthe-dark materials being charged by exposure to light, and oldstyle cathode ray tubes, coated with zinc sulfide, glowing briefly
after being switched off.
Phosphorescence in sensing
A sensor selectively detects a substance in a sample, as shown
in Figure 5. Organic molecules show this behaviour, but only
at temperatures below 100 K, in the absence of oxygen, and the
light only lasts a nanosecond (10 –9 s). Organometallics can do
the same but under normal external conditions with a glow
that is a million times longer lasting. Clearly this is of much
greater use.
Iridium is the transition metal of choice, as not only
are its colours intense, but they are particularly long lived.
FUrTHEr rEaDINg
y
x
dx
2– 2
y
Figure 3 Shapes of d orbitals
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y
x
dz
2
If you would like to read a more in-depth explanation of the
development of iridium complexes as sensors and their comparison
to ruthenium complexes, take a look at:
www.tinyurl.com/6aqvc9x
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glossary
Bidentate Literally ‘two teeth’; a ligand that has two separate
bonds with the central metal ion.
3+
(a)
Complex ion A complex ion has a metal ion at its centre with a
number of other molecules or ions surrounding it, attached to the
central ion by dative covalent (coordinate) bonds, e.g. [Fe(H2O)6]2+.
N
N
N
Ir
N
Dative bond A bond in which both bonding electrons come from
the same atom (also known as a coordinate or semipolar bond).
N
N
Delocalised electrons Electrons that are not confined to a
particular bond, instead being spread out over two or more bonds.
Electron (electronic) configuration The arrangement of
electrons in shells and orbitals around an atom.
3+
(b)
Ground state The lowest stable energy state of a system (e.g. an
atom or molecule).
Orbital A region of space around an atomic nucleus in which an
electron can be found. There are five d orbitals, each of which can
be occupied by a maximum of two electrons.
Organometallic A compound (or complex) in which carbon atoms
or organic groups are bound to metal atoms or ions.
Oxidation state (or oxidation number). The difference (positive
or negative) between the number of electrons associated with an
element in a compound and the element itself (elements have an
oxidation state of zero). Complexes of transition metal elements are
often able to have a range of oxidation states.
Phosphorescence The process of gradually releasing absorbed
energy as light. This occurs when electrons return from an excited
state (higher energy level) to the ground state.
Tridentate ‘Three teeth’; a ligand with three separate bonds with
the central metal ion.
Sensor
White light in
Low intensity,
short-lived glow emitted
No test molecule present
White light in
Higher intensity, or
different decay time or
different colour emitted
Test molecule present
Figure 5 Schematic explanation of how a sensor molecule works
Iridium is used in its oxidation state of +3, giving it an electron
configuration of d6 as explained earlier. Two tridentate ligands
form with N–Ir bonds (Figure 6a). This complex is amended
(Figure 6b) to turn it into a sensor by making it a Lewis base
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Chemistry_21_2_press.indd 22
N
Ir
N
N
N
N
(c)
Emission intensity (507 nm)
ligand An atom, ion or molecule that is bonded via a dative
covalent bond to a metal ion, usually a transition element. All
ligands have active lone pairs of electrons in the outer energy level,
which are used to form the dative bonds with the metal ion.
N
N
200
150
100
50
0
1
2
3
Source: Gareth Williams, Durham University
4
5
6
7
pH
Figure 6 (a) The organo-iridium complex, with one Ir3+ ion bonded
to six nitrogens in two tridentate ligands. (b) The sensor complex,
which can donate a lone pair of electrons to accept a proton
(H+). (c) Plot showing the intensity of the emission of the iridium
complex at different pH values
(i.e. it can donate an electron pair). When H+ is absent (i.e.
high pH), the material glows intensely red. As soon as H+ is
introduced (i.e. the pH value falls), the glow is much duller and
lasts for less time. This change can easily be tracked and is a
simple method for following pH variations (Figure 6c).
Beyond pH
While it is interesting to track pH via organo-iridium complexes,
chemical indicators and electronic pH meters already exist.
The really exciting developments are in biochemical fields. In
May 2011 Sergey Borisov at Graz University, Austria, reported
work on Ir(iii) complexes with bidentate ligands (Figure 7).
With this arrangement of ligands there are two remaining slots
Chemistry Review
29/09/2011 10:53
Box 2
Problems caused by low blood-oxygen levels
Oxygen deficiency in tissues is related to:
• tumour growth
• retinal damage from diabetes
• rheumatoid arthritis
Hypoxia (low blood-oxygen levels) is a potential problem with:
• altitude sickness on long-haul flights, causing potentially fatal
complications
• diving underwater, e.g. by breathing mixtures of gases with low
oxygen content
• premature babies and their neonatal development
HEMERA TECHNOlOGIES
IMAGEDJ
Oxygen deficiency in tissues can
aggravate rheumatoid arthritis
Hypoxia can affect premature babies’ neonatal development
for different ligands. This allows the Ir(iii) complex to be made
soluble in both polar and non-polar solvents, as well as being
incorporated into inert materials, such as polystyrene.
The complexes change colour depending on the oxygen
content of the surroundings. In solution the complex acts as a
probe for dissolved oxygen levels; on an inert surface it acts as
a trace oxygen sensor. Since oxygen is critical for the healthy
functioning of many processes in living cells, detecting oxygen
content in cells can be a vital diagnostic tool for many different
conditions (Box 2). In all these cases, the clear change in colour
of the Ir(iii) complex could be a life-saver.
L
N
N
Ir
N
N
L
Figure 7 Ir(III) complex with vacancies for two further ligands
(marked l)
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In 2009, Jason McNeill at Clemson University reported that
he had been able to link colour sensors with that most modern
of materials: the nanoparticle. Under normal conditions, the
sensors glow intensely red. When oxygen is introduced, the
phosphorescence dulls. The more oxygen present, the more
the phosphorescence is reduced. So, as a person’s oxygen level
drops the nanoparticles would glow redder and redder: the ideal
warning sign.
At present, McNeill has only worked with iridium’s sister
element, platinum, but given the results reported elsewhere that
iridium has superior colour-changing properties when compared
to platinum, the development of Ir nanoparticle indicators
looks likely. It seems appropriate that a metal discovered by the
son of a clergyman may have a key role to play in saving lives.
David Lewis gained a chemistry degree from the University of York
in 1976. After a career in science education, his most recent post
was as hidden heritage education officer for Groundwork in Selby.
In this role he has investigated one of Selby’s most prestigious sons,
Smithson Tennant FRS, discoverer of iridium.
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