Chemical Kinetics in Amine Containing
Monodentate and Bidentate Cobalt Ligands
Andrew McTammany
Cl
H3N
CoIII
H3N
NH2
Cl
Cl
Cl
NH3
NH3
Cl
+ H2O 
H3N
NH3
H2O
+ Cl-
NH3
Cl
CoIII
+ H2O 
H2N
CoIII
NH2
H2N
CoIII
NH2
H3N
H2O
H2N
NH2
H2N
+ Cl-
Background
Dichloride Cobalt (III) compounds can be
arranged in either a cis or trans configuration.
The isomerization reaction can be studied
between the two isomers.
By varying the ligands attached to the central
cobalt atom, the changes in the isomerization
kinetics can be elucidated.
Background
Temperature dependent NMR spectroscopy can
be used to monitor the kinetic parameters of
the aquation reaction between the trans and
cis isomers of both compounds. Using line
widths, absorption can be determined.
trans-Co(C2H8N2)2Cl2 + H2O cis-[Co(C2H8N2)2 (H2O)Cl]Cl2
trans-Co(NH3)4Cl2 + H2O cis-[Co(NH3)4(H2O)Cl]Cl2
Background
By varying the temperatures at which the kinetic
data is obtained, the following equation can
be used to determine the enthalpy and
entropy of activation.
Ln( )
An Eryling plot of ln(k/T) versus 1/T yields this
formula.
Background
Elucidating the thermodynamic properties of
the two cobalt compounds, molecular orbital
theory can be employed to account for
relative stabilities of these compounds.
Method
Trans dichloro-bis-ethylenediamine cobalt (III)
was synthesized from CoCl2*6H2O. It was
mixed 10% ethylenediamine before being
heated in an evaporation dish to before 12M
HCl was added to form the desired green
crystals.
CoCl2*6H2O + 2C2H8N2 + HCl  trans-[Co(C2H8N2)2Cl2 ]Cl
Method
[Co(NH3)4CO3]NO3 is first produced as an
intermediate by adding (NH4)2 CO3 to Cobalt
(II) Nitrate hexahydrate in concentrated
ammonia. 30% hydrogen peroxide was then
added slowly to the precipitate.
NH3, (NH4)2 CO3 , H2O2
Co(NO3)2*6H2O  [Co(NH3)4CO3]NO3
Method
Concentrated HCl was then added to
[Co(NH3)4CO3]NO3 to produce the trans
crystals in a temperature dependent reaction.
[Co(NH3)4CO3]NO3 + 3 HCl  trans-[Co(NH3)4Cl2]Cl + CO2 +HNO3
Results
The formation of trans-[Co(C2H8N2)2Cl2 ]Cl was
successful, producing 72.4%, but attempts to
take temperature dependent NMR spectra
were less fruitful. Spectroscopic data from
UV/Vis analysis provided a rate constant at
room temperature for evaluation.
+ H 2O 
+ ClNH2
Cl
H2N
NH2
CoIII
CoIII
NH2
Cl
H2N
Cl
H2O
H2N
NH2
H2N
Results
First Order Isomerization Kinetics
0.5605
0.56
0.5595
0.559
0.5585
0.558
0.5575
0.557
0.5565
0.556
0.5555
-Ln(A)
Absorption
Absorbtion vs Time
0
50
100
150
Time (sec)
200
250
0.588
0.587
0.586
0.585
0.584
0.583
0.582
0.581
0.58
0.579
0.578
y = 4E-05x + 0.5792
0
50
100
150
Time (sec)
The reaction demonstrated first order kinetics, with a rate
constant of 3.6*10 -5 ± .4*10-5 sec-1
200
250
Results
Only the cis-Co(NH3)4Cl2 was produced in 45.3%
yield, and not the trans product while
following the procedure in volume 31 of
Inorganic Syntheses by Erdman.
Results
A second trial was performed, altering the
procedure and conducting the experiment in
acetone instead of water. This would lower
the reaction temperature and eliminate the
possibility of the aquation reaction. A blue
product was produced and characterized using
NMR spectroscopy.
Discussion
The rate found was not the true isomerization,
but racemization, as substantial quantity of
the initial trans isomer was still in solution,
shown by the final UV/Vis spectrum. The
value of 3.6*10 -5 ± .4*10-5 sec-1 compares
somewhat comparably with the literature
value of 4.9*10 -4 sec-1.
Discussion
The reasoning behind the failed formation of
trans-Co(NH3)4Cl2 has to do with the
formation of the intermediate,
[Co(NH3)4CO3]NO3. This compound is in a cis
conformation as shown in the diagram.
O
O
H3N
H3N
CoIII
NH3
C
O
NH3
Discussion
Amines form particularly strong bonds with
Cobalt (III), d6, compounds. A molecular
orbital diagram demonstrates this. It only
occupies the t2g orbital, whereas a Co(II)
compound would have unpaired electrons.
t1u*
a 1g*
eg*
t2g
__ __ __
__
__ __
__ __ __
Discussion
Since Co(NH3)4CO31+ is cis, the presence of heat
and excess chloride ions prevented the
displacement of an amino group for a Cl,
instead just displacing the CO3. This would
result in the cis conformation.
O
O
H3N
H3N
CoIII
NH3
Cl
C
O
NH3
+HCl 
H3N
H3N
CoIII
NH3
Cl
NH3
Discussion
The trans was formed when the experiment was
conducted in acetone with diluted HCl.
However, after prolonged heating, the
compound isomerized into the cis
conformation.
O
O
H3N
H3N
CoIII
NH3
C
O
+ HCl 
Cl
H3N
H3N
NH3
CoIII
Cl
NH3
NH3
+ heat 
Cl
H3N
H3N
CoIII
NH3
Cl
NH3
Conclusion
To elucidate the kinetic and thermodynamic
quantities, the experiment needs to be
repeated. The trans-Co(NH3)4Cl2 should be
synthesized using a lower temperature and
lower concentration of acid. A mixture of HCl
and H2SO4 could be used instead. From there
the effect of different ligands can be
evaluated.
Conclusion
The experiment did demonstrate the stability of
Cobalt (III) compounds with amine ligands. It
is interesting to note that the trans[Co(C2H8N2)2Cl2 ]Cl was readily formed. This
suggests that the energy of activation into the
cis conformation is higher, since it was not as
easily produced experimentally.
References
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Orvis, Jeffery A. Journal of Chemical Education. 2003; 80, 803-805
Angelici, R.J. Synthesis and Techniques in Inorganic Chemisty; Saunders: Philadelphia, 1969; pp 25-30.
Borer, L. L. ; Erdman, H.W. In Inorganic Syntheses; Cowley, A.H., Ed; Wiley: New York, 1997; Vol 31, pp 270271.
Bailar J.C. In Inorganic Syntheses; Wiley: New York, 1946 Vol 2 pp 222-225.
Shriver, Atkins, Langford. Inorganic chemistry; Oxford University Press: Oxford, 1994, pp 242-243
Holleman-Wiberg. Inorganic chemistry; Academic Press: San Diego , CA, 2001; p 1484
Sargeson, AM. Aust. J. Chem., 1963, 16, 352-5
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A special thanks to Wednesday’s Inorganic Chemistry Lab Section