This document provides instructions for preparing and analyzing several coordination compounds containing oxalate ligands. Students will prepare one of four complexes - potassium tris(oxalato)chromate(III) trihydrate, potassium bis(oxalato)copper(II) dihydrate, potassium tris(oxalato)iron(III) trihydrate, or potassium tris(oxalato)aluminum(III) trihydrate. The preparation involves the reaction of metal salts with oxalic acid and other reagents. Students will collect and analyze their product to determine the percent yield. They will also observe reactions and color changes of cis and trans isomers of chromium oxalate complexes.

1. Preparation and
Reactions of
Coordination
Compounds: Oxalate
Complexes
To gain some familiarity with coordination compounds by preparing a rep-
resentative compound and witnessing some typical reactions.
Apparatus
balance
Bunsen burner and hose
8-oz wide-mouth bottle
No. 6 two-hole rubber stopper
glass stirring rod
9-cm filter paper
wire gauze
vial
Chemicals
cis- and trans-K[Cr(C204h(H20h]
(prep given)
6MNH3
KzC204 · H20 (potassium
oxalate monohydrate)
acetone
ferrous ammonium sulfate
6MH2S04
ice
9-cm Buchner funnel
250-mL suction flask
aspirator
100-mL and 250-mL beakers
thermometer
ring stand and iron ring
glass wool
aluminum powder
6MKOH
H2C204 (oxalic acid)
KzCr207 (potassium dichromate)
50% ethanol
95% ethanol
absolute alcohol
CuS04 · 5(H20)
6%H202
6MHC1
When gaseous boron trifluoride, BF3, is passed into liquid trimethylamine,
(CH3)3N, a highly exothermic reaction occurs, and a creamy white solid,
(CH3)3N: BF3, separates. This solid, which is an adduct of trimethylamine
and boron trifluoride, is a coordination compound. It contains a coordinate
covalent, or dative, bond uniting the Lewis acid BF3 with the Lewis base
trimethylamine. Numerous coordination compounds are known, and in fact
nearly all compounds of the transition elements are coordination com-
pounds wherein the metal is a Lewis acid and the atoms or molecules joined
to the metal are Lewis bases. These Lewis bases are called ligands, and the co-
ordination compounds are usually denoted by square brackets when their
formulas are written. The metal and the ligands bound to it constitute what
is termed the coordination sphere. In writing chemical formulas for coordi-
nation compounds, we use square brackets to set off the coordination sphere
from other parts of the compound. For example, the salt NiC12· 6H20 is in
From Laboratory Experiments, Tenth Edition, John H. Nelson and Kenneth C. Kemp. Copyright
© 2006 by Pearson Education, Inc. Published by Prentice Hall, Inc. All rights reserved.
77
Experiment
OBJECTIVE
APPARATUS
AND CHEMICALS
DISCUSSION

2. Preparation and Reactions of Coordination Compounds: Oxalate Complexes
2+ 3-
0
II
/c..'c=o
0::--. 0 I
"c-0~1,, I ~P
I '·co···'"
..<:c-o"" I ~o
or 0 '
'c,.,,C=O
II
0
hexaaquanickel(II) trioxalatocobaltate(III)
1- 1-
O~ OH2 -90
C-0 I 0-C
I
1111
''·cr "'''*' I
c-o"" I ~o-c""
0-9 OH2
"O
cis-diaquadioxalatochromate(III) trans-diaquadioxalatochromate(III)
A FIGURE 1 Typical octahedral (6-coordinate) coordination compounds.
reality the coordination compound [Ni(H20)6]Cl2, with an octahedral geom-
etry, as shown in Figure 1.
The apexes of a regular octahedron are all equivalent positions. Thus, each of
the monodentate (one donor site) H20 molecules in the [Ni(H20)6]
2+ ion and
the three bidentate (two donor sites) oxalate ions, C20 4
2-, in [Co(C20 4)3]2- are
in identical environments. The water molecules in the two isomeric com-
pounds, cis- and trans-[Cr(C20 4h(H20)ir, are in equivalent environments
within each complex ion (coordination compound), but the two isomeric ions
are not equivalent to one another. The two water molecules are adjacent in the
cis isomer and opposite one another in the trans isomer. These two isomers
are termed geometric isomers, and although they have identical empirical and
molecular formulas, their geometrical arrangements in space are .different.
Consequently, they have different chemical and physical properties, as your
laboratory instructor will demonstrate through the reactions shown in
Figure 2.
Your goal in this experiment is to prepare an oxalate-containing coordina-
tion compound. It may be analyzed for its oxalate concentration. You will
prepare one of the following compounds:
1. K3[Cr(C204)3] · 3H20
2. K1[Cu(C204)i] · 2H20
78

3. Preparation and Reactions of Coordination Compounds: Oxalate Complexes
~-~
0
:~--_.,p diluteNH3 ~-Q--~
0
:____.,po=c/ // ~-----/cll _o o=c/ // '-. ~----//cll _o
(/----- / - (/----- / -
d""-- - -, /___c~0 dilute HCI d""-- - -, /-C~0
OH2 OH2
trans isomer (rose-violet) hydroxo complex (light brown)
sp. soluble
0 0
II II
~~aJ-~~~iluteNH3 ~9J~~o
H2~,</ dilute HCl H2~,</
C=O C=OI I
0-C 0-C
11 II
0 0
cis isomer (purple-green) dichroic hydroxo complex (deep green)
very soluble
~ FIGURE 2 Reaction of geometric isomers: cis- and trans-[Cr(C204)z(H20hr ions.
The procedure is to place a small amount of an aqueous solution of each com-
plex on separate pieces of filter paper on a watch glass. Let them dry, and then
add a drop of reagent. Observe the results.
3. K3[Fe(C204)3] · 3H20
4. K3[Al(C204)3] · 3H20
Your laboratory instructor will tell you which one to prepare. Each of these
compounds will be prepared by someone in your lab section so that you can
compare their properties. (CAUTION: Oxalic acid is a toxic compound and
is absorbed through the skin. Should any come in contact with your skin,
wash it offimmediately with copious amounts ofwater.)
Prepare one of the complexes whose synthesis is given below.
1. Preparation of K3[Cr(C20 4)3] • 3H20
K2Cr207 + 7H2C204 · 2H20 + 2K2C204 · H20 ~
2K3[Cr(C204)3] · 3H20 + 6C02 + l7H20
Slowly add 3.6 g of potassium dichromate to a suspension of 10 g of oxalic
acid in 20 mL of H20 in a 250-mL beaker. The orange-colored mixture should
spontaneously warm up almost to boiling as a vigorous evolution of gas
commences. When the reaction has subsided (about 15 min), dissolve 4.2 g of
potassium oxalate monohydrate in the hot, green-black liquid and heat to
boiling for 10 min. Allow the beaker and its contents to cool to room temper-
ature. Add about 10 mL of 95% ethanol, with stirring, into the cooled solu-
tion in the beaker. Further cool the beaker and its contents in ice. The cooled
liquid should thicken with crystals. After cooling in ice for 15 to 20 min, the
79
PROCEDURE

4. Preparation and Reactions of Coordination Compounds: Oxalate Complexes
Rapid water
flow into sink
9-cm Buchner funnel
Disconnect from the aspirator
to remove suction
.A. FIGURE 3 Suction filtration assembly.
250-mL filter
flask
crystals should be collected by filtration with suction using a Buchner funnel
(See Figure 3) and filter flask. Wash the crystals on the funnel with three 10-
mL portions of 50% aqueous ethanol followed by 25 mL of 95% ethanol and
dry the product in air. Weigh the air-dried material and store it in a vial. You
should obtain about 9 g of product. Calculate the theoretical yield and deter-
mine your percentage yield. Reactions of chromium(III) are slow, and your
yield will be low if you work too fast.
actual yield in grams
% yield = 100 x ----------
theoretical yield in grams
Your instructor may tell you to save your sample for analysis in Experiment 37.
EXAMPLE 1
In the preparation of cis-K[Cr(C204)z(H20h] · 2H20, 12.0 g of oxalic acid
was allowed to react with 4.00 g of potassium dichromate, and 8.20 g of
cis-K[Cr(C20 4h(H20h] · 2H20 was isolated. What is the percent yield in
this synthesis?
SOLUTION:
K2Cr207 + 7H2C204 · 2H20 ~ 2K[Cr(C204h(H20h] · 2H20
+ 6C02 + l3H20
From the above reaction, we see that 1 mole of K2Cr20 7reacts with 7 moles of
H2C204 to produce 2 moles of K[Cr(C204h(H20h] · 2H20. In our synthesis we
have used the following:
4.00 g
moles K2Cr207 =
294
.19 g/mol = 0.0136 mol
12.0 g
80

5. Preparation and Reactions of Coordination Compounds: Oxalate Complexes
Our reaction requires a 7:1 molar ratio of oxalic acid to K2Cr20 7, and we
have actually used a 6.999 molar ratio or, within experimental error, the
stoichiometric amount of each reagent. Hence, the number of moles of
K[Cr(C204h(H20h] · 2H20 formed should be twice the number of moles
of K2Cr20 7 reacted.
moles K[Cr(C204h(H20h] · 2H20(expected) = (2)(0.0136 mol) = 0.0272 mol
The theoretical yield of K[Cr(C204h(H20h] · 2H20 is
(0.0272 mol)(339.2 g/mol) = 9.23 g
Our percentage yield is then
(100)(8.20 g)
% yield = = 88.8%
(9.23 g)
2. Preparation of K2[Cu(C20 4)2] • 2H20
CuS04 · SHzO + 2K2C204 · H20 ~ Kz[Cu(C204)z] · 2H20
+ KzS04 + SH20
Heat a solution of 6.2 g of copper sulfate pentahydrate in 12 ml of water
to about 90°C and add it rapidly, with vigorous stirring, to a hot ("'90°C)
solution of 10.0 g of potassium oxalate monohydrate (K2C20 4· H20) in 50
mL of water contained in a 100-mL beaker. Cool the mixture by setting the
beaker in an ice bath for 15 to 30 min. Suction-filter the resultant crystals
using a Buchner funnel (see Figure 3) and filter flask and wash the crystals
successively with about 12 mL of cold water, then 10 mL of absolute ethanol,
and finally 10 mL of acetone, and air dry. Weigh the air-dried material and
store it in a vial. You should obtain about 7 g of product. Calculate the theo-
retical yield and determine your percentage yield. Your instructor may tell
you to save your sample.
3. Preparation of K3[Fe(C20 4)3] • 3H20
(NH4)z[Fe(H20)z(S04)z] · 6H20 + H2C204 •2H20 ~
FeC204 + H2S04 + (NH4)zS04 + 10 H20
H2C204 · 2H20 + 2FeC204 + 3K2C204 · H20 + H202 ~
2K3[Fe(C204)3] · 3H20 + H20
This preparation contains two separate parts. Iron(II) oxalate is prepared
first and then converted to K3[Fe(C204h] ·3H20 by oxidation with hydro-
gen peroxide, H20 2, in the presence of potassium oxalate.
Prepare a solution of 10 g of ferrous ammonium sulfate hexahydrate in
30 mL of water containing a few drops of 6M H2S04 (to prevent premature
oxidation of Fe2+ to Fe3+ by 0 2 in the air). Then add, with stirring, a solu-
tion of 6 g of oxalic acid in 50 mL of H20. Yellow iron(II) oxalate forms.
Carefully heat the mixture to boiling while stirring constantly to prevent
bumping. Decant and discard the supernatant liquid and wash the precipi-
tate several times by adding about 30 mL of hot water, stirring, and decant-
ing the liquid. Filtration is not necessary at this point.
To the wet iron(II) oxalate, add a solution of 6.6 g of K2C20 4· H20 in
18 mL of water and heat the mixture to about 40°C. SLOWLY AND CAU-
TIOUSLY add 17 mL of 6% H20 2, while stirring constantly and maintaining
the temperature at 40°C. After the addition of H20 2 is complete, heat the
81

6. Preparation and Reactions of Coordination Compounds: Oxalate Complexes
mixture to boiling and add a solution containing 1.7 g of oxalic acid in 15 ml
of water. When adding the oxalic acid solution, add the first 8 mL all at once
and the remaining 5 ml dropwise, keeping the temperature near boiling.
Remove any solid by gravity filtration and add 20 mL of 95% ethanol to the
filtrate. Cover the beaker with a watch glass and store it in your lab desk
until the next period. Filter by suction using a Buchner funnel and filter flask
(Figure 3) and wash the green crystals with a 50% aqueous ethanol solution,
then with acetone, and air-dry. Weigh the product and store it in a vial in the
dark. This complex is photosensitive and reacts with light according to the
following:
In order to demonstrate this, place a small specimen on a watch glass near
the window and observe any changes that occur during the lab period. You
should obtain about 8 g of product. Calculate the theoretical yield and deter-
mine your percent yield. Your instructor may tell you to save your sample.
Keep it away from light by wrapping the vial with aluminium foil.
4. Preparation of K3[ Al(C20 4h] •3H20
Al+ 3KOH + 3H2C20 4· 2H20 ~ K3[Al(C20 4)3] •3H20 + 6H20 + ~H2
Place 1 g of aluminum powder in a 200-mL beaker and cover with 10 mL of
hot water. Add 20 ml of 6 M KOH solution in small portions to regulate the
vigorous evolution of hydrogen. Finally, heat the liquid almost to boiling in
order to dissolve any residual metal. Maintain the heating and add a solution
of 13 g of oxalic acid in 100 mL of water in small portions. During the neutral-
ization, hydrated alumina will precipitate, but it will redissolve at the end of
the addition after gentle boiling. Cool the solution in an ice bath and add 50
mL of 95% ethanol. If oily material separates, stir the solution and scratch the
sides of the beaker with your glass rod to induce crystallization. Suction-filter
the product using the Buchner funnel (see Figure 3) and suction flask and
wash with a 20-ml portion of ice-cold 50% aqueous ethanol and finally with
small portions of absolute ethanol. Dry the product in air, weigh it, and store
it in a stoppered bottle. You should obtain about 11 g of product. Calculate the
theoretical yield and determine your percent yield. Your instructor may tell
you to save your sample.
Preparation of Materials for the Demonstration
(These preparations should be done a week before the laboratory period.)
cis-K[Cr(C204h(H20h] • 2H20
K2Cr207 + 7H2C20 4· 2H20 ~
2K[Cr(C204)z(H20)z] · 2H20 + 6C02 + 13H20
Separately powder in a dry mortar 12 g of oxalic acid dihydrate and 4 g of
potassium dichromate. Mix the powders as intimately as possible by gentle
grinding in the mortar. Moisten a large evaporating dish (10 cm) with water
and pour off all the water but do not wipe dry. Place the powdered mixture
in the evaporating dish as a compact heap; it will become moistened by the
water that remains in the evaporating dish. Cover the evaporating dish with
a large watch glass and warm it gently on a hot plate. A vigorous sponta-
82

7. Preparation and Reactions of Coordination Compounds: Oxalate Complexes
neous reaction will soon occur and will be accompanied by frothing, as
steam and C02 escape. The mixture should then liquefy to a deep-colored
syrup. Pour about 20 mL of 95% ethanol on the hot liquid and continue to
gently warm it on the hot plate. Triturate (grind or crush) the product with a
spatula until it solidifies. If complete solidification cannot be effected with
one portion of 95% alcohol, decant the liquid, add another 20 mL of 95% al-
cohol, warm gently, and resume the trituration until the product is entirely
crystalline and granular. The yield is essentially quantitative at about 9 g.
This compound is intensely dichroic, appearing in the solid state as almost
black in diffuse daylight and deep purple in artificial light.
trans-K[Cr(C20 4h(H20)i] •3H20 Dissolve 12 g of oxalic acid dihydrate in
a minimum of boiling water in a 300-mL (or larger) beaker. Add to this in
small portions a solution of 4 g of potassium dichromate in a minimum of
hot water and cover the beaker with a watch glass while the violent reaction
proceeds. After the addition is complete, cool the contents of the beaker and
allow spontaneous evaporation at room temperature to occur so that the so-
lution reduces to about one-third its original volume (this takes 36 to 48
hours). Collect the deposited crystals by suction filtration and wash several
times with cold water and 95% alcohol and air-dry. The yield is about 6.5 g.
The complex is rose-colored with a violet tinge and is not dichroic.
Waste Disposal Instructions All oxalate- and metal-containing solutions
should be disposed of in appropriate containers.
Before beginning this experiment in the laboratory, you should be able to
answer the following questions:
1. Define the terms Lewis acid and Lewis base.
2. Define the terms ligand and coordination sphere.
3. Define and give an example of a coordination compound.
4. Define the term geometric isomer.
5. Draw structures for all possible isomers of the six-coordinate com-
pounds [Co(NH3)4Cl2] and [Co(NH3)3Cl3].
6. Are the chlorine atoms in equivalent environments in each oft.he com-
pounds [Co(NH3)4Cl2] and [Co(NH3)JCl3]?
7. What is the meaning of dichroism?
8. What is the meaning of trituration?
9. Look up the preparation of an oxalate complex of Ni, Mn, or Co. Cite
your reference and state whether this preparation would be suitable to
add to this experiment. Why or why not?
10. Find an analytical method to determine the amount of Fe, Cu, Cr, or Al
in your oxalate complex. Cite the reference to the method. Could you
do the determination with the chemicals and equipment available in
your laboratory? Why or why not?
11. Oxalic acid is used to remove rust and corrosion from automobile radi-
ators. How do you think it works?
83
PRE LAB
QUESTIONS

8. NOTES AND CALCULATIONS
84

9. Date----------- Laboratory Instructor - - - - - - - - - - - - - - - - - -
REPORT SHEET
Preparation and Reactions
of Coordination Compounds:
Oxalate Complexes
1. Complex prepared - - - - - - - - -
2. Chemical reaction for its preparation - - - - - - - - - - - - -
3. Theoretical yield of oxalate complex (show calculations):
4. Experimental yield of oxalate complex ______
5. Percent yield of oxalate complex (show calculations):
6. Color and general appearance of complex:
EXPERIMENT
7. Describe the reactions of cis- and trans-K[Cr(C20 4h(H20)z] with NH3 and the reverse reactions with
HCl using chemical equations. List any observations, such as color changes or apparent solubilities.
8. Is your complex soluble in H20? _____ Alcohol? _____ Acetone?
85

10. Report Sheet • Preparation and Reactions of Coordination Compounds: Oxalate Complexes
QUESTIONS
1. Sodium trioxalatocobaltate(III) trihydrate is prepared by the following reactions:
[Co(H20)6]Cl2 + KzC204 · H20 ~ CoC204 + 2KC1 + 7H20
2CoC204 + 4H20 + H202 + 4Na2C204 ~ 2Na3[Co(C204)3] · 3H20 + 2NaOH
What is the percent yield of Na3[Co(C20 4h] · 3H20 if 7.6 g is obtained from 12.5 g of [Co(H20)6]Cl2?
2. Why are K3[Cr(C204)3] · 3H20, Kz[Cu(C204)z] · 2H20, and K3[Fe(C204)3] · 3H20 colored, whereas
K3[Al(C20 4h] ·3H20 is colorless?
3. What are the names of the following compounds?
K3[Cr(C204)3] · 3H20
Kz[Cu(C204)z] · 2H20
K3[Fe(C204)3] · 3H20
K3[Al(C204)3] · 3H20
4. What is the percent oxalate in each of the following compounds?
A. K3[Cr(C204)3] · 3H20
B. K2[Cu(C204)z] · 2H20
C. K3[Fe(C204)3] · 3H20
D. K3[Al(C204)3] · 3H20
86

11. Answers to Selected
Pre Lab Questions
3. A coordination compound is formed by the reaction of a Lewis acid
with a Lewis base; it contains one or more coordinate covalent bonds.
An example is the compound H3N:BF3.
4. Geometric isomers have the same empirical and molecular formulas,
but differ in their spatial arrangements of the constituent atoms.
5. The compounds [Co(NH3)4Cl2] and [Co(NH3)3Cl3] can each exist in
two geometric isomeric forms, which are shown below.
Cl Cl
;~"'~-~NH, '/J~Cl~c~// ~c~//
H:iN-----,~3 H:iN-----,-NH3
Cl NH3
trans
meridional
cis
Cl
H~--1---~Cl// _)c ------ /'
~ ~/
H:iN-----,-Cl
NH3
facial
6. The chlorine atoms are in equivalent environments in cis- and trans-
[Co(NH3)4Cl2] and facial [Co(NH3)3Cl3], but there are two different
chlorine environments in meridional [Co(NH3)3Cl3].
7. Dichroism is the property according to which the colors of a crystal are
different when the crystal is viewed in the direction of two different
axes.
8. Triturate means to cause a semisolid to become a solid by crushing or
grinding.
9. You must find the answer to question 9 in the library.
10. You must find the answer to question 10 in the library.
11. It works by forming a soluble iron(II) oxalate complex, [Fe(C20 4)3]4-, by
reacting with iron oxide (rust), according to the reaction: FeO +
3H2C204 ~ H4[Fe(C204)3] + HzO. In the reactions of Fe203 or
Fe30 4, iron is also reduced by oxalate and C02is formed.
87