EXPERIMENT 1 Visible Spectroscopy - The Spectrochemical Series Background A char
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EXPERIMENT 1 Visible Spectroscopy - The Spectrochemical Series
Background
A characteristic of transition metal compounds is their variety of colours. Study of the origin
of these colours has been central to the development of an understanding of the electronic
structure and many related properties of transition metal ions. The primary determinants of
the colour of a particular metal ion compound are the nature and spatial distribution
(stereochemistry) of the ligands attached to the metal. Spectroscopy may therefore be used to
establish such properties of a metal complex and an important current application of this
procedure is in the deduction of the nature of the active site of metalloenzymes. Quantitative
measurements and a very large number of simple transition metal compounds have shown that
their spectra may be grossly described in terms of three types of absorption band:
(i) Bands with extinction coefficients (e) < 1. Such bands are important in relation to some
systems with special spectral properties (e.g., the ruby laser), but frequently remain
undetected in measurements of ordinary solution spectra. They are weak because the
electron undergoing excitation changes its spin and such processes are, to a first
approximation, forbidden.
(ii) Bands with extinction coefficients in the range 10-200. These bands are responsible for
the blue colour of the hydrated Cu2+ ion, the red colour of Cr3+ in ruby and the yellow
colour of Co(NH3)6
3+ , for example, and are usually referred to as "d-d" or "ligand field"
absorptions. They arise from electronic excitations, without change of spin, within the
manifold of metal d orbitals, the degeneracy of which has been lifted by the presence of
ligands bound to the metal ion. Because the electronic transitions occur between orbitals,
which have (for most complex ion stereochemistries) the same symmetry with respect to
inversion about the nucleus ("gerade" or "ungerade") they are forbidden under the
so-called Laporte Selection Rule. However, several mechanisms operate to weaken this rule and the absorptions are usually therefore 1-2 orders of magnitude more intense than
the "spin-forbidden" type (1).
(iii) Bands with extinction coefficients >1000. Such absorptions originate in various ways,
including excitation of an electron localised in a metal ion orbital to a ligand localised
orbital (or vice versa). These are known as "charge transfer" transitions and they are
involved in much of the photochemistry and photo-electrochemistry of transition metal
compounds.
The object of the present experiment is to study some absorption spectra where bands of
type (ii) are prominent.
Cobalt(III) spectroscopy
The Co3+ ion contains 6 electrons in its outer (3d) valence shell. When six ligands are
attached to the metal ion to form an octahedral complex CoL 6
3+ , the d orbital electrons do not
all experience identical repulsions from the ligand donor atom lone pairs. An electron in an
orbital directed along the bond axes ?dz2,d x2-y2) experiences greater repulsion than one in
an orbital (d d d ) xy xz yz , , directed "between" the bonds. This can be formalised by saying that
in an octahedral environment metal ion d orbitals are split into a lower energy group of three
and a higher energy group of two. The energy difference is a measure of repulsion between
metal and ligand electrons or the "ligand field" and is an important quantity in various
theoretical analyses (where it is referred to as lODq, Do , etc.).If the ligand field splitting energy is greater than interelectronic repulsion, electrons will tend to pair up and occupy the lower d subshell. This is the case for almost all Co(IlI) complexes i.e. the lowest energy configuration is diamagnetic "spin paired" d2 xy d2 xz d2 yz. "d-d"
transitions in Co(III) complexes occur when an electron from the degenerate dxy, dyz, dxz set is excited to either the dz
2 or dX2y 2 orbital. Since there are 3 equivalent occupied orbitals a transition actually consists of 3 superimposed (degenerate) excitations. Because interelectronic repulsions are not identical in the excited states, however, there are actually two sets of three d-d transitions possible in octahedral Co(III). The lower energy set is
(dxy)2 (dxz)2 (dyz)2 --------(dxy)l (dxz)2 (dyz)2 (dx2-y2)l
(dxy)2 (dxz)2 (dyz)2 ----------- (dxy)2 (dxz)l (dyz)2 (dx2-y2)l
(dxy)2 (dxz)2 (dyz)2 --------------(dxy)2 (dxz)2 (dyz)l (dx2-y2)l
and for a simple complex such as Co(NH3) 63+ two bands with extinction coefficients near
70 M-l cm-l are seen in the visible spectrum.For a series of complexes such as Co(NH3)5Xn+ the values of l may serve as a crude estimate of the differences in ligand field strength between X and NH3. The ordering of ligands
thereby achieved is termed the Spectrochemical Series.The object of the present experiment is to establish some basic features of transition metal ion spectra and to define the Spectrochemical Series for several simple ligands bound to Co(III).To do this the complexes Co(NH3)5Xn+ , X = N-3, Cl-, NO-3, NO-2, OH-, OH2, and NH3, will be prepared and their spectra in the visible region, 320-700 nm recorded. The preparative chemistry of Co(III) is usually uncomplicated, but purity of material is essential for spectroscopic measurements and hence the experiment is important as an exercise in basic preparation and purification techniques.Measurement of Visible Spectra
A convenient concentration for measurement of the visible spectra of the above complexes,
using a l cm path length is 5 x 10-3 mol L-l. Calculate the relative molar mass of each
complex, assuming the salt obtained to be anhydrous, and from this calculate the amount
required to prepare 100 cm3 of 5 x 10-3 mol L-1 solution. Use distilled water made slightly
acidic by the addition of a few drops of glacial acetic acid as solvent for the hexammine,
nitropentammine, chloropentammine, and nitratopentammine complexes. The spectrum of the
solution of the nitratopentammine complex must be recorded as soon as possible after
preparation to avoid inaccuracies due to hydrolysis. For the aquopentammine complex, two
spectra should be run. The first is for [Co(NH3)5OH2](C1O4)3 dissolved in 1 mol L-1 HCl and
the other for [Co(NH3)5OH2](ClO4)3 initially dissolved in aqueous ammonia (1 mol
L-1, 5 mL) and then made up to volume with distilled water. The former is the true aquo
complex spectrum, the latter the hydroxopentamminecobalt (III) spectrum. Weigh out the
samples of complex salts on the 4 decimal place balance and prepare the solutions in 100 mL
volumetric flasks.Your demonstrator will instruct you in the use of the UV-VIS Spectrophotometer. The ratio of
transmitted intensities is usually displayed logarithmically as the quantity termed Absorbance,
since this is simply related to properties of the absorbing system through the Beer-Lambert
law.
First run a blank/background on water. Then fill the sample cell with the cobalt complex
solution. Position the cell in the spectrophotometer and record the spectrum over the
range 325 to 700 nm. Repeat for the rest of the solutions. Always rinse the sample cell at
least once with some of the solution to be measured before filling to avoid mixing or dilution
with other solutions. Check that the faces of the cell are clean and dry for each measurement.
Determine Al values at the lmax for each spectrum and use these to calculate el values at
maxima (e max) from the Beer-Lambert law. From your lmax values place the ligands H2O,
NH3, Cl-, NO2? , OH- and N03- in order of increasing ligand field strength on cobalt (III).
REPORT
Basic spectroscopic results as below should be tabulated as lmax (n.m). A and e (L mol ?1 cm?1) for the
various complexes and as the Spectrochemical Series for the ligands X (of Co(NH3)5Xn+). The
following questions should also be answered.
QUESTIONS
(i) "Spin forbidden" (e < 1) bands?
(ii) High intensity ("charge transfer") bands?
(iii) Check with a text or reference source for the generally accepted sequence of ligands in
the Spectrochemical Series. Comment on any differences between this and your
experimentally determined sequence.
(iv)What is the purpose of the glacial acetic acid?
(v) Why not use a strong acid such as bench HCl?
Explanation / Answer
Answer:
(i) Bands with extinction coefficients (e) < 1 are the forbidden bands. These are weak because the
electron undergoing excitation changes its spin and such processes are, to a first approximation, forbidden.
(ii) Bands with extinction coefficients >1000 are the high intensity charge transfer bands.
(iii) H2O, NH3, Cl-, NO2 , OH- and NO3- in order of increasing ligand field strength on cobalt (III).
In the given series Cl- ligand is weaker than H2O and NH3 ligands. And so as with the ligands NO3- and OH-.
The actual increasing order will be:
Cl- < NO3-< OH- < H2O < NH3< NO2
(iv) Glacial acetic acid is useful as a solvent for the complexes containing hexammine, nitropentammine, chloropentammine, and nitratopentammine.
(v) Using strong acid like HCl will release weak Chloride ligand (Cl-) which destabilizes the complex molecules.
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