Title:
Chromium (III) Complexes
Intro:
This experiment involved the preparation of chromium (III) complexes. The cis/trans
isomeric pair provided an opportunity to look at isomerism in co-ordinate chemistry.
Procedure:
(i)
The preparation of potassium trans-diaquabis(oxalato)chromate(III) trihydrate.
12gf of oxalic acid dihydrate was dissolved dissolved in a 400cm3 beaker in
the minimum amount of boiling water. To this a potassium dichromate
solution was added made by dissolving 4g of potassium dichromate in the
minimum amount of water. The potassium dichromate was added in small
portions due to the violent reaction. When the reaction was completed the
dark coloured solution was left to crystallise in a large beaker covered with a
watch glass.
(ii)
Preparation of cis-K [ Cr (C2 O4)2 (H2 O)2 ]. 2H2O
Again, 12g of oxalic acid dihydrate was taken with 4g of potassium
dischromate and they were ground into fine powders in the fume hood. Once
mixing in the mortar was complete they were transferred to a 150cm3 beaker
containing about 5 drops of water. The damp powder was then covered with a
watch glass before a vigorous reaction occurred. Large volumes of carbon
dioxide were evolved until only the dark coloured solution remained. 20cm3
of alcohol was added to the solution which was being continually warmed.
After stirring the solution, a crystalline product eventually resulted. The
product was filtered with a Buchner funnel, washed with alcohol and dried in
the air. The yield was recorded.
(iii)
9g of oxalic acid was once more dissolved in the minimum amount of water at
about 70C. 3g of powdered potassium dichromate was added slowly and in
small portions. The solution was boiled and 3.5g of potassium oxalate were
added after all of the carbon dioxide had evolved. The mixture was allowed to
cool before 4cm3 of ethanol was added to it. It was then placed in an ice-bath.
The crystalline product was obtained using the Buchner funnel. It was washed
with 10cm3/10cm3 solution of ethanol and water respectively and then with
just ethanol. It was recrystallised from an ethanol/water mixture and washed
and dried.
Pre-practical questions:
1. (i) trans – K[Cr ( C2O4)2 (H2O)2 ] . 3 H2O
(ii)
cis - K[Cr ( C2O4)2 (H2O)2 ] . 2 H2O
(iii)
K3[Cr ( C2O4)2 ] . 3 H2O
2.
A chiral complex has a non-superimposable mirror image. Optical Isomers are
cis – K[Cr ( C2O4)2 (H2O)2 ] . 2 H2O and K3[Cr ( C2O4)2 ] . 3 H2O.
3.
The crude products must be washed with ethanol before drying because the alcohol itself
is a dehydrating agent. It also gets rid of organic soluble impurities.
4.
The term identate describes a ligan on which there are two points of attachment. (Binds to
a metal ion via two bonds e.g. (C2O4)2- ).
Post-practical questions:
1.
Oxalic acid acted as the reducing agent in the syntheses. Optical activity can be generated
only if it is possible to resolve a chiral compound into enantiomers. Enantiomers rotate
the plane of plane polarised light in opposite directions by equal amounts. Resolution of
chiral complexes is only achieved if the system is kinetically inert so that the process of
ligand substitution is slow compared with the time taken to resolve the complex. For
example the inert chiral complex [Co ( C2O4)3]3- ( d6, low spin) can be resolved but the
labile complex [Fe ( C2O4)3]3- ( d5, high spin) cannot. Resolution of complexes
generally involves the presence of chelating ligands. Optically active cations can be
resolved by reacting the mixture with an optically active anion to generate
diastereoisomeric salt (different physical properties ).
2.
(a)
When HCl is used, quantitative conversion into the chlorochromate ion occurs
CrO3(OH)- + H+ + Cl-  CrO3Cl- + H2O
NH3 and OH- act as bases and ligands. The NH3 can convert the species to oxaletohyoloxo complex. Upon acidification this change would be reversed. Furthermore, the
oxalate could be protonated. This will result in a colour change – ligand substitution can
occur upon acidification because the protonated ox would be a weakened ligand.
(b)
3.
[Cr(C2O4)6]9- is not obtained because of the chromium complex there is no metal-metal
bonding and the magnetic moment is that expected for a d 3 configuration. There is static
repulsions between the negative changes of OX, and also complexes in which the ligands
act as bidentate are more stable.
--------------------------Paul Walsh – 2001
pwalsht@maths.tcd.ie