The Preparation and Properties of Organocobaloxime Complexes
Version: 14 September 2001
Dr John Maher, School of Chemistry, Bristol University
firstname.lastname@example.org Tel (928)7653
Just as the tetrahedron is an icon of organic chemistry, so the octahedron is an icon of
inorganic chemistry, and because Co(III) complexes are easy to study and prepare, a very large
proportion of our knowledge of inorganic isomerism, reaction mechanisms, and the general
properties of octahedral complexes is based upon studies of Co(III) chemistry.1 In Co(III)
complexes the cobalt atom shows an affinity for nitrogen donor ligands, moreover these are
mostly substitution inert i.e. non-labile. The slow substitution rates made the complexes
amenable to study before the development of modern instrumental methods. In Co(I) and Co(II)
complexes the ligands are labile (rapidly substituted), thus the normal mode of preparation for
Co(III) complexes is to form a Co(II) or Co(I) version of the required complex, then to oxidise it to
its Co(III) equivalent.
The dimethylglyoxime monoanion (dmgH- ), a strong chelate ligand, forms Co(III)
complexes [Co(dmgH)2AX], A = NH3, py, PPh3; X = Cl-, Br-, I-, NO2-, SCN-, N3-, R-. Thus A is a
neutral N, P, O, or S donor Lewis base ligand, and X is a mono-anion; X can also be a carbanion,
R-, giving organometallic complexes such as [CH3Co(dmgH)2A] . Referred to as a cobaloxime
complex, the latter compound can be called ‘methylcobaloxime’. Cobaloxime complexes in
oxidation states Co(I) to Co(IV) are known.
CH 3 CH 3
O N N OH
A fascinating development was the discovery in 1962, from an X-ray crystal structure
determination by Dorothy Hodgkin, that the Vitamin B 12 coenzyme contained a Co-C -bond.2
Chemical studies have since shown that this bond is non-labile yet at the same time a relatively
weak bond. The discovery of the alkylcobaloximes occurred at about the same time, so that they
were adopted at an early stage as cobalamin (Vitamin B 12) models.
The chelate ligands around the cobalt(III) can vary, so that large numbers of
organocobalt complexes have now been prepared and studied, both as models for B 12
chemistry, and in their own right as interesting compounds. In this experiment you will prepare,
and then study some properties of one of the alkylcobaloximes. 3
R O O
Co CH 3
R = alkyl group, B = Lewis base.
Preparative Section. (Must be carried out by each student individually)
Whilst carrying out this preparation record in your laboratory notebook your observations about
the progress of the preparation at its various stages :
• the colour changes that take place in the reaction mixture.
• any general observations on the course of the reactions
• any difficulties/problems encountered with the preparation
• your % yield
Quantities needed for the preparation
Quantity Moles Compound
0.860 g (0.0075 mole) Dimethylglyoxime.
0,890 g (0.00375 mole) Cobalt(II) chloride hexahydrate.
0.30g - Sodium Hydroxide (A).
0.15g - Sodium Hydroxide (B)
0.29g (0.3ml) (0.00375 mole) Pyridine
0.020g (0.000604 mole) Sodium Borohydride
Calculate this (0.01mole) Haloalkane
20 ml - Methanol.
Apparatus (See photograph and diagram in laboratory)
A 50ml 2-neck flask is fitted with a pressure-equalising funnel, N2 bubbler and small
magnetic stirrer piece. The 2-neck flask should be clamped by its neck, the clamp should not be
attached to the dropping funnel. The flask will eventually be placed in a small plastic cooling
bath over the centre of the magnetic stirrer table. The top of the funnel is connected to the
nitrogen bubbler. Apply a very small amount of Vaseline to the apparatus joints. After mixing
the Co(II), the ligand and the methanol, the reactions must be conducted under nitrogen, and for
formation of the Co(I) cobaloxime, at a temperature of about -10oC.
The quantities used are small so that you should be careful when measuring, in
particular the relative amounts of the chemicals are important – small variations seem to lead to
large variations in the product yield. Except for the sodium borohydride (weighed on a 4-figure
balance) the other chemicals can be weighed with a top-pan 3 figure balance. Use plastic
weighing boats for solids (not paper), various small plastic capped vials , measuring cylinders
and Pasteur pipette droppers are provided for handling the liquids.
Weigh the Co(II) salt, transfer it into the 2-neck flask using the powder funnel. Remove
the powder funnel, and add half of the methanol, stir until the Co(II) salt is dissolved. Weigh the
dimethylglyoxime, add this to the flask contents, followed by the remaining methanol (to wash
Close the flask with the pressure-equalising dropping funnel and stopper, attach the
nitrogen bubbler, keep stirring and pass nitrogen gently through the suspension for about 10
minutes. From now on, the remaining reagent solutions must be added to the dropping funnel
by removing the nitrogen tube briefly and using a Pasteur pipette to put the reagent at the
bottom of the dropping funnel .
Sodium hydroxide pellets are difficult to measure in small amounts, so that it is easier to
work from a stock solution in water (a 15% w/v NaOH solution is provided), you will need 2 ml
and 1ml quantities of this solution. Measure the pyridine carefully into a small vial, and dilute
with 2-3 ml of methanol.
Put the 2 ml NaOH (Solution A) together with the pyridine solution into the dropping
funnel, making sure that the nitrogen is kept running, then slowly, drop-by-drop, add the
mixture to the cobalt and dimethylglyoxime, stirring all the time.
When the addition is finished you should have a deep brown-yellow air-sensitive
solution, possibly with a slight precipitate. All of the Co(II) and most of the dimethylglyoxime
should be dissolved up, there should be no coagulated purple or white pieces at the bottom of the
Prepare a -10oC cold bath by mixing ice (200g) with about 50 ml of ethanol. Leave the
flask to stir for about 10 minutes in the cooling mixture. Weigh the sodium borohydride very
carefully into a small glass vial, add to it 1 ml of the NaOH (Solution B ), transfer this to the
dropping funnel. Slow the rate of nitrogen bubbling slightly. Add the NaOH/NaBH 4 solution
to the Co(II) mixture. Stir for about a minute. Weigh the haloalkane into a vial, transfer this to
the dropping funnel, wash the vial with a couple of mls of methanol and add this also. Add the
haloalkane/methanol slowly over a minute or so from the dropping funnel to the (now) Co(I)
solution. Stir for about 10 minutes. Remove the low temperature bath and bring the reaction
flask back up to room temperature. Stop the nitrogen.
Extraction of Your Product
Warm your product in the 2-neck flask very gently on a steam bath until the methanol is
just off the boil. CAUTION METHANOL VAPOUR IS POISONOUS – DO NOT LEAVE
BOILING METHANOL FLASKS ON THE STEAM BATH. In any case boiling your product
will decompose it! Place a fluted filter paper in the larger plastic powder funnel. Put a couple of
mls of methanol into a single neck 100 ml conical flask. Put the funnel with the fluted paper in the
neck of this flask and place the flask and funnel on the steam bath, when the methanol at the
bottom of the flask is boiling, and the flask and filter system are thoroughly hot, filter your (hot)
product solution through the fluted paper. Try to stop the filtrate boiling. With this technique the
filtration is conducted under methanol vapour, nevertheless filtration may be rather slow, so be
patient! If there is some solid left, then this may dissolve up in a little more hot ethanol, if it is
insoluble, then it is an unwanted cobaloxime by-product. In view of the small quantities be
careful with your washings and your product, it is easy to get it 'spread out' at this stage, and
also easy to knock the flasks over on the steam bath.
Transfer the filtrate to a round bottom flask and evaporate the filtrate with a rotary
evaporator, keeping the flask warm with hot water on a steam bath. CAUTION :ALWAYS
WEAR SAFETY SPECTACLES WHEN WORKING WITH OR NEAR THE ROTARY
EVAPORATOR. Be sure that the flask that you use is not more than one third full - this will help to
prevent bumping. Divide the product solution into two parts if necessary. You need to
evaporate the methanol/water until a thick muddy mass is obtained - not to dryness.
Precipitate/crystallise the product.
Add 50ml of ice cold water containing 1% by volume of pyridine to the round bottom
flask. Put a stopper on it and shake the flask vigorously to dislodge any solid from the flask
walls. Allow the crystals to 'improve' for 15-20 minutes in the ice bath. Filter the product
through a glass sinter. Dry your sample carefully, drying is crucial to the stability of the
product. The sample is best dried by gently drawing air through the crystals on the sinter. Be
patient and do not try to stir the contents of the sinter with your spatula whilst it is wet, this will
break up the precipitate, slow down the drying process, and you may even get glass from the
sinter in your product!
Weigh your product. Calculate the yield. Hand in your sample in a small vial. Your
sample bottle must be labelled with : Your Name ; Date; Amount of product; Product Name,
e.g.Samantha Plunkett, Dec 2001, 0.95g, pentyl-cobaloxime.
AFTER YOU FINISH, PLEASE CLEAN AND DRY ALL OF YOUR
APPARATUS FOR THE NEXT PERSON DOING THIS EXPERIMENT.
Tests and Reactions.
Carry out the following tests, make careful observations. Use the various spectra provided for 5)
Each student should do test 1). Tests 2) and 3) may be done jointly.
1) Reaction of Co(II) with hydroxide. Make up a dilute solution of cobalt(II) chloride in water
(approx. 10mg/ml). Add drops of 2M sodium hydroxide from a Pasteur pipette. Shake the
solution. Watch what happens over a few minutes. Record your observations.
2) Solvent effects on Co(II) solutions. Make up 2 mls of a solution of cobalt(II) chloride in
ethanol in a test tube. Now cool the test tube with some liquid nitrogen. You don't need to
freeze the tube! Warm the tube to room temperature again. Record your observations.
3) Photolysis of methylcobaloxime. Make a saturated solution of the methylcobaloxime
complex (available from the hatch), in about 3 ml of water containing a couple of drops of
concentrated hydrochloric acid, filter this into a B14 stoppered test tube. Bubble N 2 for a few
minutes to expel the air, seal the tube quickly with a greased B14 stopper. Support the tube on a
clamp stand, leave this in the window to expose the solution to sunlight. When there is very
bright sunshine the reaction can be over in minutes, normally the tube will need to be left an hour
or even much longer depending on the sunlight level. The effects of photolysis are best
observed if the tube is not moved at all. Record your observations of any changes to the solution.
4) Photolysis - Spin Trapping , an Electron Magnetic Resonance (EMR) example. Spin
trapping is a technique in which a reactive free radical reacts with a double bond of a
diamagnetic compound, the spin trap, to form a less reactive and stable free radical, the spin
adduct. The technique is used when the primary free radical cannot be detected directly because
it is very reactive, and so has a lifetime of microseconds or less. By trapping the radical, the
lifetime can be increased to minutes or longer. The example EMR spectrum (at the end of the
script) was obtained by spin trapping an aqueous solution of the methylcobaloxime complex,
exposed in strong sunlight together with the spin trap PBN, N-tert-Butyl--phenylnitrone. The
trapping reaction is:
O R O
CH N C(CH 3 ) 3 + R . C .
N C(CH 3 ) 3
5) UV/visible spectra measurements of cobaloxime solutions. Visible spectra, between 300nm
and 600nm, of solutions of the methylcobaloxime have been measured:
a) Methylcobaloxime complex, aqueous solution only - nothing added.
b) Methylcobaloxime complex, aqueous solution + concentrated hydrochloric acid.
c) Methylcobaloxime complex, aqueous solution + excess of pyridine .
d) Methylcobaloxime complex, in 1,2-dichloroethane solution.
Spectra diagrams are labelled a) to d) an shown at the end of the script Look at these carefully
and answer the questions given later.
Write-up and Questions
You must write your report independently, even though some of the work has been done in
collaboration with others. Please keep your answers brief and to the point.
• you are not required to re-write the experiment notes!
• you should keep these instruction sheets with your write-up as a reference.
• you must include your observations from the notes that you took during the
preparation, and the observations that you made during the tests.
• you must quote your preparation yield (% efficiency to within 1%)
• read the additional experiment notes in the library TP659 to help understand the
1.Write a short description of your observations during the preparation. In these give the
reactions which are occurring during your preparation, indicating the oxidation state of the
cobalt for the different reaction stages. Try to correlate this with your observations about the
colour of the solution at the various stages. You can assume the following stages to the
a) Dissolving the Co(II) chloride in the methanol and adding the ligand
b) Under N2, adding the NaOH and the pyridine.
c) Cooling the solution and adding more NaOH and NaBH 4
d) Adding the alkyl halide
Note that the Co(II) dimethylglyoxinate complex, probably [Co(dmgH) 2(MeOH)(py)], does not
form until step b), it is not formed until the ligand is converted to its anion with NaOH.
2. Suggest a mechanism for the formation of the alkylcobaloxime assuming that the cobalt is in
the Co(I) state. What is the alternative mechanism if a Co(II) ) dimethylglyoxinate complex is
involved? Hint: Co(I) is a powerful nucleophile and low spin Co(II) behaves like a free radical.
The mechanisms also control the reaction yield.
The Cobalt(II) Tests
3. Describe and explain what happened when you added NaOH solution to the cobalt(II)
4. Describe and explain what happened when you dissolved the cobalt(II) complex in ethanol,
and then cooled and warmed it again.
The photolysis and spin-trapping
5. When the photolysis is carried out with a strong solution and under nitrogen, you should
observe the formation of a light pink solution and a white precipitate; gas bubbles may also be
seen to form. However the rate at which all of this occurs is very dependent on the sunlight
level! On a cloudy day the photolysis is very slow. Also, if the photolysis is conducted in air the
solution colour becomes light yellow, the precipitate still forms, but no radicals can be trapped
from the solution, neither are gas bubbles formed.
a) What evidence do you have for the formation of free radicals by the photolysis?
b) Suggest inorganic and organic products for the photo-decomposition. Assume that the light
photon cleaves the Co-C bond homolytically. Note that alkyl radicals do not react readily with
surrounding water molecules.
c) Why does the spin-trap not work in the presence of atmospheric oxygen? (No spin adduct is
detected by EMR)
d) The photo-decomposition only occurs in strong sunlight, a dull cloudy day inhibits photo-
decomposition, photolysis is a fine weather experiment! Moreover the reaction takes place
behind window glass. What do these facts and the UV/visible spectra tell you about the range
of wavelengths of light inducing photo-decomposition, i.e.give the wavelength range over which
you think the complex is sensitive (in nanometers, nm). Calculate the energy involved with the
upper and lower limits for the wavelength range (in kJ.mol-1) Hint: observe the wavelengths over
which the complex absorbs light, and the region over which window glass transmits light.
e) Very briefly, explain the observed splitting pattern in the EMR spectrum, estimating the
electron-nucleus hyperfine couplings AN and AH (like J couplings in NMR spectra) between the
nitrogen, 14N has I=1, and the hydrogen, 1H has I=½, on the adjacent carbon. If hyperfine
couplings were observed from the methyl and/or the tertiary butyl group hydrogens, to the
electron, describe the EMR splitting pattern that you would you expect to see? Hint: remember
NMR and the Pascal triangle.
Addition of pyridine or acid, and the UV/visible spectra.
6. Suggest what is happening to the solutions when you add more pyridine on the one hand, or
acid on the other. Remember that you add extra pyridine at the end of the preparation; this
observation helps to explain why extra pyridine is added at this stage. Hint: compare the
spectra carefully and contrast a) and b) with c) and d). The purpose of preparing solution d) in
1,2 dichloroethane is to show what the spectrum of the complex is in a non-co-ordinating solvent.
Hint: Trans effect.
7. What is the valence electron count of the metal centre in the complexes [Co(CH3)(dmgH)2py],
[Co(dmgH)2(MeOH)(py)] and [Co(dmgH)2py]? Hint: 18e rule.
1. 'Hexol', Molecule of the Month, Sept 1997, via
2. 'Vitamin B12', Molecule of the Month, May 1997. via
3. Worsley Library Tutorial Paper TP 659 contains some précis notes from sections in Cotton
and Wilkinson, and which relate to the background chemistry for this system.
4. TP659 and these notes, and are shown on the Web at