Transcript TM shape and colour

Transition metal complexes:
colour
Transition metal complexes
Objectives:
• Describe common
shapes of transition
metal complexes
• Explain why transition
metal complexes are
coloured
Outcomes:
• Revise understanding
of the terms complex
and ligand
• Describe the shape of
some specific transition
metal complexes
• Explain why solutions
appear coloured
• Use a visible spectrum
to deduce the colour of
a complex ion in
solution
CO-ORDINATION NUMBER & SHAPE
the shape of a complex is governed by the number of ligands around the central ion
the co-ordination number gives the number of ligands around the central ion
a change of ligand can affect the co-ordination number
Co-ordination No.
Shape
Example(s)
6
Octahedral
[Cu(H2O)6]2+
4
Tetrahedral
[CuCl4]2-
2
Square planar
Pt(NH3)2Cl2
Linear
[Ag(NH3)2]+
Predict the shape of the following
complexes
– [Co(NH3)5Cl]2+
– [Cr(H2O)4Cl2]+
– [FeCl4]-
Check your understanding
• For each of the following complexes, give the charge on the central
metal ion and its coordination number and its name
–
–
–
–
–
–
–
–
[Co(NH3)6]3+
[Cu(NH3)4(H2O)2]2+
CuCl42[Fe(CN)6]4K4[Fe(CN)6]
Na2CoCl4
[Co(NH3)6]Cl3
[Cu(CN)2]-
• What shape is each of the complex ions likely to be?
• Define each of the following terms, using the complex [Ni(CN)4]2- to
illustrate your answer:
– Ligand
– Coordination number
– Shape
Complexes and colour
• Complex formation is often accompanied
by a change in colour.
• Example: Adding ammonia to aqueous
copper (II) sulphate: Light blue to deep
blue colour change
• Q: How does colour arise?
• A solution appears coloured because
certain wavelengths of visible light are
absorbed by the solution
Colour
blue and green
not absorbed
white light
a solution of copper(II)sulphate is blue because
red and yellow wavelengths are absorbed
Absorbed
colour
λ (nm)
Observed
colour
λ (nm)
Red
Orange
Yellow
Green
650
600
550
500
Green
Blue
Dark blue
Red
500
450
430
650
Blue
Violet
450
400
Yellow
Green-yellow
600
560
COLOURED IONS
a solution of copper(II)sulphate is blue because
red and yellow wavelengths are absorbed
COLOURED IONS
a solution of copper(II)sulphate is blue because
red and yellow wavelengths are absorbed
COLOURED IONS
a solution of nickel(II)sulphate is green because
violet, blue and red wavelengths are absorbed
Visible spectroscopy (Absorption
spectroscopy)
• A spectrophotometer measures the absorption of light at different
wavelengths
Quartz mirror splits
light beam in two
Light source
M
Sample
PM
Blank
PM
Analyser
M = monochromator: selects wavelength
PM = photomultiplier: converts light into electric current
Analyser: Compares two electric currents. Any
difference is dependent on the absorption of light by the
sample
Recorder: pen traces absorption spectrum
Recorder
What happens when light is
absorbed?
• Electrons are excited to a higher energy
level
What happens when light is
absorbed?
INCREASING ENERGY /
DISTANCE FROM
NUCLEUS
4f
4
IRON
4d
4p
3d
4s
3
3p
1s2 2s2 2p6 3s2 3p6 4s2 3d6
In an isolated atom or ion, the isolated d orbitals have
the same energy
What happens when light is
absorbed?
xy
xz
yz
There are 5 different orbitals of the d variety
x2-y2
z2
3d ORBITALS
SPLITTING OF 3d ORBITALS
Placing ligands around a central ion causes the energies of the d orbitals to change
Some of the d orbitals gain energy and some lose energy
In an octahedral complex, two (z2 and x2-y2) go higher and three go lower
In a tetrahedral complex, three (xy, xz and yz) go higher and two go lower
OCTAHEDRAL
TETRAHEDRAL
3d
3d
Degree of splitting depends on the
CENTRAL ION
and the
LIGAND
•The energy difference between the d orbitals corresponds to the frequency of visible
light.
•The energy difference between the levels affects how much energy is absorbed when
an electron is promoted. The amount of energy governs the colour of light absorbed.
What happens when light is
absorbed?
Theory
•
ions with a d10 (full) or d0 (empty) configuration are colourless
(there needs to be at least 1 electron which can be excited AND there
needs to be an empty d orbital which can be occupied when the electron is
excited)
•
ions with partially filled d-orbitals tend to be coloured
•
it is caused by the ease of transition of electrons between energy levels
•
energy is absorbed when an electron is promoted to a higher level
•
the frequency of light is proportional to the energy difference
ions with
or
d10 (full) Cu+,Ag+ Zn2+
d0 (empty) Sc3+ configuration are colourless
e.g. titanium(IV) oxide TiO2 is white
colour depends on ...
transition element
oxidation state
ligand
coordination number
Plenary
• Describe two common shapes of transition
metal complexes
• Explain why solutions are coloured in
terms electrons
• What does the colour of a transition metal
complex depend upon?