Chemical Kinetics in Amine Containing Monodentate and Bidentate Cobalt Ligands Andrew McTammany + H 2 O  + 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(C 2 H 8 N 2 ) 2 Cl 2 + H 2 O  cis-[Co(C 2 H 8 N 2 ) 2 (H 2 O)Cl]Cl 2 trans-Co(NH 3 ) 4 Cl 2 + H 2 O  cis-[Co(NH 3 ) 4 (H 2 O)Cl]Cl 2

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 CoCl 2 *6H 2 O. It was mixed 10% ethylenediamine before being heated in an evaporation dish to before 12M HCl was added to form the desired green crystals. CoCl 2 *6H 2 O + 2 C 2 H 8 N 2 + HCl  trans-[Co(C 2 H 8 N 2 ) 2 Cl 2 ]Cl

Method [Co(NH 3 ) 4 CO 3 ]NO 3 is first produced as an intermediate by adding (NH 4 ) 2 CO 3 to Cobalt (II) Nitrate hexahydrate in concentrated ammonia. 30% hydrogen peroxide was then added slowly to the precipitate. NH 3, (NH 4 ) 2 CO 3, H 2 O 2 Co(NO 3 ) 2 *6H 2 O  [Co(NH 3 ) 4 CO 3 ]NO 3

Method Concentrated HCl was then added to [Co(NH 3 ) 4 CO 3 ]NO 3 to produce the trans crystals in a temperature dependent reaction. [Co(NH 3 ) 4 CO 3 ]NO HCl  trans-[Co(NH 3 ) 4 Cl 2 ]Cl + CO 2 +HNO 3

Results The formation of trans-[Co(C 2 H 8 N 2 ) 2 Cl 2 ]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 2 O  + Cl -

Results The reaction demonstrated first order kinetics, with a rate constant of 3.6*10 -5 ±.4*10 -5 sec -1

Results Only the cis-Co(NH 3 ) 4 Cl 2 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(NH 3 ) 4 Cl 2 has to do with the formation of the intermediate, [Co(NH 3 ) 4 CO 3 ]NO 3. This compound is in a cis conformation as shown in the diagram.

Discussion Amines form particularly strong bonds with Cobalt (III), d 6, compounds. A molecular orbital diagram demonstrates this. It only occupies the t 2g orbital, whereas a Co(II) compound would have unpaired electrons. t 1u * __ __ __ a 1g * __ e g * __ __ t 2g __ __ __

Discussion Since Co(NH 3 ) 4 CO 3 1+ is cis, the presence of heat and excess chloride ions prevented the displacement of an amino group for a Cl, instead just displacing the CO 3. This would result in the cis conformation. +HCl 

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. + HCl  + heat 

Conclusion To elucidate the kinetic and thermodynamic quantities, the experiment needs to be repeated. The trans-Co(NH 3 ) 4 Cl 2 should be synthesized using a lower temperature and lower concentration of acid. A mixture of HCl and H 2 SO 4 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(C 2 H 8 N 2 ) 2 Cl 2 ]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 Orvis, Jeffery A. Journal of Chemical Education. 2003; 80, Angelici, R.J. Synthesis and Techniques in Inorganic Chemisty; Saunders: Philadelphia, 1969; pp Borer, L. L. ; Erdman, H.W. In Inorganic Syntheses; Cowley, A.H., Ed; Wiley: New York, 1997; Vol 31, pp Bailar J.C. In Inorganic Syntheses; Wiley: New York, 1946 Vol 2 pp Shriver, Atkins, Langford. Inorganic chemistry; Oxford University Press: Oxford, 1994, pp Holleman-Wiberg. Inorganic chemistry; Academic Press: San Diego, CA, 2001; p 1484 Sargeson, AM. Aust. J. Chem., 1963, 16, A special thanks to Wednesday’s Inorganic Chemistry Lab Section