|
Appendix
4. Electron
configurations and the theory and variation of complex ion colour
-
For the 3d-block know the
complete order of filling of the sub-shells from Z=21 to 30 and be
able to write out the full or abbreviated electron
configuration.
-
See under each
element and even more detail in Electron Configurations in Periodic Table section 2.2
-
Transition metal electron
arrangements are listed
-
The number of
orbitals per sub-shell, 1 for s, 3 for p, and 5 for d sub-shell.
-
PLEASE watch out for the two
‘quirks’ for Cr and Cu atoms and the order of
electron removal when forming positive ions e.g. for the 3d block of
transition metals, you remove the 4s electrons first, before any
of the 3d electrons.
-
Transition metals can be
identified by the colour of their complexes which of course is a
very characteristic feature of their chemistry (e.g. the
hydroxide precipitates which
are, of course, all neutral complexes).
-
The colour
can varies with
change in (i) oxidation state, (ii) ligand and (iii) co-ordination
number or shape (which in turn depends on the ligand and oxidation
sate).
-
All of these three
factors are linked to the electronic state of the central metal ion, so,
if the electronic levels are changed, the difference between quantum
levels changes, therefore the wavelength of the light photons absorbed
changes, i.e. the observed colour changes e.g.
-
e.g. (i) The
same ligand (H2O), shape and co-ordination number but
different oxidation state.
-
e.g. (ii) The
same oxidation state, shape and co-ordination number but
different ligand.
-
e.g. (iii) The
same oxidation state but with a different ligand, shape and
co-ordination number.
-
[Cu(H2O)6]2+,
blue hexaaquacopper(II) ion and [CuCl4]2-,
yellow tetrachlorocuprate(II) ion.
-
Same oxidation
state +2, but different ligands (water and chloride ion), different
shape (octahedral and tetrahedral) and different co-ordination
number (6 and 4).
-
COLOUR
THEORY for transition element complexes: The argument is
presented from the point of view of an octahedral complex, but
similar arguments apply for a tetrahedral or square planar
complex.
-
There are five 3d
sub-shell orbitals whose 3D spatial representations are shown
below. Theoretically it is considered that the ligands in an
octahedral complex approach the central metal ion along the x, y
and z axis, which would minimise the repulsion between the
orbitals of bonding electrons in the six M-ligand dative covalent bonds (note that
4s and 4p orbitals are involved in complex ion bonding).
The
electronic ground states
of scandium(III), titanium(III), copper(II) and zinc(II) are illustrated below.

The
electronically excited states of titanium(III) and copper(II) are
illustrated below.
-
The colour arises from
electronic transitions from the ground state to excited states, the
energy needed can be calculated using
-
Planck's
Equation, ΔE = hv
, E = energy of a single photon (J), h
= Planck's Constant (6.63 x 10-34 JHz-1),
v = frequency (Hz).
-
Therefore if the
photo energy/frequency is equal to
ΔE
then energy is absorbed and an electron can be promoted from the
lower 3d level to the higher 3d level.
-
If
ΔE
is in the visible light frequency range the complex will be
'coloured'.
-
In the case
of coloured transition metal complexes, the colour arises from
visible light energy absorption on promoting electrons from the
lower 3d levels to the higher 3d levels.
-
However, this can only
occur if there is at least one electron in the 'lower' 3d orbitals and
at least one half-filled 'higher' 3d quantum level, i.e. the minimum pre-conditions for
an electronic transition or 'excitation'.
-
Consequently because
there a lack of such
possible transitions in Sc(III) and Zn(II) their compounds are usually
colourless i.e. no light absorbed in the visible region of the spectrum
-
In the true transition metals
from Ti to Cu,
it is possible for the electromagnetic radiation energy to produce this excitation from the lower to the higher
3d sub-levels and it is usually in the visible region. The frequencies of visible
radiation are absorbed and the perceived colour arises from the frequencies not
absorbed.
-
The electronic structure
and colour of some typical 'simple' aqueous ions is shown below. They
are all hexa-aqua ions of an octahedral shape except ...
-
The colour you see in a
transition metal compound is the visible light that isn't absorbed by
the 3d electronic transitions. For example, copper(II) complexes often
absorb in the red area of the visible spectrum, so the resulting
colour observed is green-blue.
-
Colour changes
in transition metal reactions can arise from change of ligand,
change in co-ordination number or change in metal oxidation state
(sometimes several of these simultaneously, see
cobalt(II) ion reactions with
ammonia + oxygen or chloride ion).
-
The colours are
quite useful for simple transition metal ion identification tests e.g.
precipitates with sodium hydroxide and ammonia (see pictures) and the
thiocyanate test for iron(III)
ions.
-
Ultraviolet and
visible absorption spectra
-
Dyes and pigments
-
Ultraviolet and visible
spectroscopy can be used to determine the concentration of metal
ions in solution, usually after the addition of a suitable ligand to
intensify the colour using the more elaborate technique of
spectrophotometry or the simpler technique of
colorimetry
- appendix 9.
-
Theory: diagram,
spectra
-
Examples:
-
Colorimetric analysis of coloured solutions for quantitative analysis
using a colorimeter is described in Appendix 9.
Scandium
* Titanium * Vanadium
* Chromium
* Manganese * Iron * Cobalt
* Nickel
* Copper *
Zinc
* Silver & Platinum
| (italian) Doc Brown
Chimica Tavola periodica revisione Note - Parte 10. 3d block
- Transition Metals 3d block - metalli di transizione
Appendice 4 configurazioni di elettroni e la teoria e la
variazione del colore di ioni complessi Appendice 4.
Electron configurazione e teoria del colore * |
Website
content copyright © Dr W P Brown 2000-2010 All rights reserved
on
revision notes, puzzles, quizzes, worksheets, x-words etc. * Copying of website
material is not permitted * I do not personally endorse the adverts -
but they do pay for the site! |