Jenalyn D. Osuna

ACID HYDROLYSIS OF trans-[CoCl2(en)2]+ I. Introduction Inorganic reactions have various forms of mechanism and the main reactions include: substitution reaction, reduction-oxidation reaction, addition-dissociation reaction, oxidative-addition or reductive elimination reactions and free radical reaction. Observed in d-metal complex reactions occurring in solutions, the solvent molecules work as ligands that compete for the central metal atom and the complex formed follows a reaction mechanism which is a substitution reaction. Such reaction happens when the ligand attached to the central metal ion is replaced by another ligand coming from an external source and the state of the central atom is unchanged. (Miessler G.L. and Tarr D.A.) Metal complexes may be classified according to the rate of their substitution reactions. A metal complex can be called labile when their half-life is less than one minute while it is inert when their halflife is greater than one minute. For metal complexes to be studied, chemical kinetics, which is also known as the study of reaction rates, is u involved. (Housecroft, C.E. and A.G. Sharpe, 2005) By studying the reaction rates of metal complexes, their mechanisms of reaction can be understood. To achieve this, variables that affect the rate of reaction such as pressure, temperature, and presence of catalyst are controlled and optimized for the most appropriate conditions. Cobalt complexes are one of the most extensively studied of coordination compounds. They generally undergo ligand substitution reaction slow enough to be studied since the ligand groups coordinated to Co3+ ion are not easily dissociated from the Co 3+ ion, thus they are not replaced easily and a substitution reaction occur at a slow rate. (Housecroft, C.E. and A.G. Sharpe, 2005) In this experiment, the kinetic study of the acid hydrolysis of trans-[CoCl 2(en) 2]+ was observed to determine the most probable mechanism for the octahedral cobalt(III) substitution reaction, also it is designed to study its temperature dependence by computing for the activation energy (Ea) and the Arrhenius frequency factor (A). II. Methodology The experiment was divided into two parts, the first part being the synthesis of the trans[CoCl 2(en) 2]+ and the second part was kinetic study of the acid hydrolysis of trans-[CoCl 2(en) 2]+. Approximately 6g of cobalt chloride hexahydrate was dissolved in a 250mL beaker then it was added with 10mL of 15% ethylenediamine solution. It was left to stand for about 10 minutes then was cooled to an ice bath. With the beaker in the ice bath, 5mL of 30% hydrogen peroxide was added a little at a time while stirring the mixture. The beaker was allowed to stand for another 10 minutes. 10mL of 12M hydrochloric acid was slowly added to the mixture after the mixture was left standing. For 30 minutes, the mixture was again allowed to stand. The mixture was evaporated until approximately onethird of the original mixture was left, it was then transferred to a new 250mL beaker being cooled in an ice bath. The solids formed were filtered out using a sintered funnel and were washed with about 10mL of 12M hydrochloric acid. The solids were purified through recrystalllization with the use of 3:1 (v/v) mixture of 12M hydrochloric acid and distilled water as the solvent. The hot solution was decanted into another beaker, and then chilled in an ice bath for half an hour. The solids that were formed decanted out and set aside as the first crop. The decantate and any solids that were left were combined and recrystallized for the second crop of crystals. Both crops of crystals were washed with 10mL of 12M hydrochloric acid and twice with 95% ethanol. III. Results and Discussion In this exercise, trans-[CoCl 2(en) 2]+ was synthesized through acid hydrolysis. The kinetic properties of the reaction such as the reaction order and its dependence on temperature were studied. Table 3.1. Observations on the synthesis of trans-[CoCl(en)2]Cl. Action Taken / Reagent Observations 6.0g CoCl 2·6H2O + H2O Wine red solution Upon addition of ethylenediamine solution (15%) Dark brown solution 10 mins. Cool in ice bath Dark brown solution Upon addition of 30% H2O2 Dark brown solution (more opaque) Upon addition of 12M HCl Dark brown solution After standing, 20-30min Dark brown solution Evaporated mixture left Dark green solution After cooling in ice bath Dark green mixture Filter using sintered funnel: Filtrate 1 Dark green crystals Residue 1 Dark green solution After washing residue with 12M HCl Dark green mixture Filter using sintered funnel: Filtrate 2 Dark green solution Residue 2 Green solids Dissolution of residue 2 with 3:1, 12M HCl:H2O Green mixture Decant hot solution: Supernatant 1 Dark green solution Residue 3 Green crystals Cool supernatant in ice-bath for 30mins Dark green solution Decantation: Supernatant 2 Green solution Residue 4 Green crystals Wash residue 4 with 12M HCl Filtrate 3 Dark green solution Residue 5 Dark green crystals Wash residue 5 with 95% Ethanol Filtrate 4 Dark green solution Residue 6 Dark green crystals Wash residue 6 with 95% Ethanol Filtrate 5 Dark green solution Residue 7 Dark green crystals Air-dried residue 7 (First Crop) Green crystals Recrystallized supernatant 2 Dark green solution After chilling for 30mins Dark green mixture Filter by suction Filtrate 6 Residue 8 Wash with 12M HCl using sintered funnel: Filtrate 7 Residue 9 Wash with 95% Ethanol Filtrate 8 Residue 10 Wash with 95% Ethanol Filtrate 9 Residue 11 Air-dried Residue 11 (2nd Crop) Green solution Green crystals Green solution Green crystals Green solution Green crystals Green solution Green crystals Green Crystals Trans-[CoCl 2(en) 2]+ was prepared by dissolution of about 6g of CoCl 2·6H2O in 12mL water followed by addition of about 20mL 15% ethylenediamine solution to the resulting mixture. The chemical equation below shows the chemical reaction that occurred during the said steps. (Chemical Equation 1) (Chemical Equation 2) Based from the above equation, the sources of central metal ion, Co2+, as well as the ligand, en, was identified to be CoCl 2·6H2O and ethylenediamine solution, respectively. These two chemical equations are displacement reactions involving Lewis acid and base pairs. In chemical equation 1, the displacement of the Cl- ion from CoCl 2·6H2O was favored since water is a stronger Lewis base than Cl ion, which means that it will have a stronger interaction with the Co 2+ ion than Cl - ion. On the other hand, ethylenediamine ligand replaces the two water ions from the [Co(H 2O) 6]2+ intermediate since it is a stronger Lewis base than water. However, there is a possibility for the formation of a side product as described on chemical equation 3, wherein all the water ligands of [Co(H 2O)6]2+ was replaced with en. This was prevented through the use of a dilute solution of ethylenediamine. (Chemical Equation 3) The next step performed was cooling the resulting mixture in an ice bath followed by the slow addition of 5mL 30% H2O2. The purpose of adding hydrogen peroxide in aqueous medium was to oxidize the central metal ion from Co 2+ to Co3+. This reaction was described on the chemical equation below: (Chemical Equation 4) The oxidation reaction was performed on [Co(en) 2(H2O) 2]2+ and not on [Co(H2O)6]2+ since the en ligand imparts more stability than the aqua ligand. Calculation of the crystal field stabilization energy confirms this stabilizing effect. The d-orbital configuration of the central metal ion due to the en ligand and water ligand is shown on Figure 3.1. Another manifestation of the en ligand stabilizing effect is the fact that eventhough the Ecell of chemical reaction 4 is negative, the oxidation of Co2+ to Co3+ was still favored resulting in the formation of the desired product. Figure 3.1. The electron distribution diagram for the central metal ion with different ligands. After the removal of the reaction vessel from the ice bath, the solution was allowed to stand for 10 minutes and about 20mL 12M HCl was slowly added to it. The mixture was then allowed to stand for another 20-30minutes. During this step, the reaction mixture was flooded with chloride ions which displaced the aqua ligand with chloride. However, this reaction is reversible knowing that water is a stronger Lewis base than Cl-. In order to promote the direction of the reaction to the product side, water was evaporated from the reaction mixture and this forced the formation of the dichloro-complex. The reaction involved is shown on chemical equation 5. This process is in accordance with the Le Chatellier’s principle which states that the system will adjust to the stressed introduced to it towards equilibrium. (Chemical Equation 5) It can be noticed from the reaction that the displacement is selective to a particular ligand. That is, only the water ligands and not the en ligands are displaced by the chloride ions. This effect can be best explained by the stability of ligands. The fact that bidentate ligands have higher stability as compared to monodentate ligands, favored displacement of the aqua ligands rather than the ethylenediamine ligands. The hot solution was then transferred into a 250mL beaker and cooled in an ice bath. Filtration was performed using a sintered funnel. Sintered funnel was used in the experiment to filter the product since the reaction mixture is highly acidic and thus may damage ordinary filter paper. The resulting product was washed with 10mL 12M HCl in order to separate the trans-isomer with the cis-isomer since the cis-isomer is soluble with 12M HCl. The product was then recrystallized using 3:1 (v/v) mixture of 12M HCl and water. Further washing of the product with ethanol removes adhered impurities to the trans-product. The overall reaction for the synthesis of trans-[Co(en) 2Cl 2]Cl is shown below: (Chemical Equ. 6) The product were green crystals of trans-[Co(en) 2Cl 2]Cl. The mass of the product was 0.5263g with a percentage yield of 21.94%. Table 3.2. Determination of trans-[CoCl 2(en)2 ]Cl yield. Parameters Mass of sintered funnel + product, g Mass sintered funnel, g Mass of product, g Theoretical yield, g Percent Yield, % Value- The second part of the exercise was about the determination of the order of the reaction with respect to [CoCl2 (en) 2]+. In order to do this, a solution of trans-[Co(en) 2Cl 2]Cl was prepared and immersed in an ice-bath. A pink solution was obtained by heating half of the amount of trans[Co(en) 2Cl 2]Cl prepared to about 75-80ºC. At this temperature, the hydrolysis is rapid and essentially quantitative in a few moments. The pink solution was cooled to room temperature and some were poured to a buret. About 5mL of green solution was placed into a small flask and placed in a cold-water bath. The pink solution was slowly added from the buret until the mixture in the flask assumes a neutral gray color when viewed against a white background. The volume of pink solution needed to produce the color gray and pink solution was determined. The R g values were then determined. Table 3.3 summarizes the obtained Rg values. The chemical reaction involved in the experiment is shown below. (Chemical Equation 7) Table 3.3. Data on the determination of Rg values. Total Volume of Conditions Green Solution, mL 5mL green solution 5 5mL green solution + 2.5mL green solution 7.5 5mL green solution + 5mL water 10 Average - Volume of Pink Solution, mL 3.6 5.5 3.2 - Rg Values- The computed values for the Rg are 0.5814, 0.5769 ang 0.6098 with an average value of 0.5894. These results showed that there was no significant changes that occurred even when the conditions were varied since the Rg values of are close to each other. There are two possible mechanisms for the ligand exchange reaction of trans-[Co(en)2Cl 2]+ to trans-[Co(en) 2Cl 2(H2O)] 2+. The two possible mechanisms are the associative mechanism and the dissociative mechanism. The first mechanism, the dissociative mechanism, is similar to an S N1 mechanism. This mechanism is a nucleophilic substitution and follows first order kinetics. In this mechanism, the coordination number of the activated complex of the transition state is lower than the initial complex. The breaking of the bond between the metal ion and the leaving group is the basis for the determination of its over-all activation energy. This mechanism is first order kinetics and typical in octahedral complexes because the metal atom is relatively crowded and the leaving group needs to make room for the entering group. (Cotton, F.A. and G. Wilkinson, 1988) The other possible mechanism for the ligand exchange is the associative mechanism which is similar to an SN2 mechanism. In this mechanism, the coordination number of the activated complexes is higher than the initial complex. The nature of the entering group is the basis for the determination of the activation energy for this mechanism. This mechanism follows a second order kinetics and it is typical of square planar tetrahedral complexes since it is easier for an entering group to enter the relatively un-crowded region of the central metal ion and thus allowing bond formation. Cotton, F.A. and G. Wilkinson, 1988) Since an SN1 and SN2 mechanism is typical for a octahedral and a square planar complexes respectively, the theoretical mechanism for the acid hydrolysis of trans-[Co(en) 2Cl 2]Cl is SN1 with a theoretical first order reaction. The rate equation for the acid hydrolysis of trans-[Co(en) 2Cl2 ]Cl can be written as: There are other factors to be considered in determining the rate of the reaction. Aside from considering the geometry of the complex as discussed above, the charge on the central metal ion must also be considered. A high positive charge favors associative mechanism due to the strong interaction between the positively charged metal center and the negatively charged ligand. Dissociative mechanism is not favorable since it is energetically difficult to remove a ligand from a high positive charge metal ion. (Housecroft, C.E. and A.G. Sharpe, 2005) The changes in the concentration of the incoming ligand must also be considered. For an associative mechanism, a high concentration of the incoming ligand will hasten the rate of the reaction. On the other hand, a high concentration of the incoming ligand will have no effect if the mechanism is dissociative. Another thing to be considered is the nucleophilicity of the incoming ligand. The more nucleophilic is the incoming ligand, the faster is the rate of the reaction. The trend below shows that order of nucleophilicity of the ligands. (Housecroft, C.E. and A.G. Sharpe, 2005) The characteristic of the leaving group should also be considered. For a dissociative mechanism, a slower rate of reaction will happen if the leaving group is high in concentration. In addition, if a poor leaving group will be present, it will also decrease the rate of the dissociative mechanism. The trend below shows the order of the leaving group with water being the best leaving group than the other ligands. (Housecroft, C.E. and A.G. Sharpe, 2005) The last thing to be considered is chelation. The presence of large chelate groups slows down the rate of the reaction due to the formation of stable complexes rendering ligands to be replaced. An increase in the extent of chelation increases the stability of the complex thus lowers reactivity. (Miessler G.L. and Tarr D.A.) The experimental order of the reaction for the hydrolysis of trans-[Co(en) 2Cl 2]+ was determined by using different equations shown below. The observations and data on the determination of the reaction order were summarized to Table 3.4. (Zero order) (First Order) (Second Order) Table 3.4. Observations and data on the determination of order of reaction with respect to [CoCl 2(en) 2]+. Color of Solution Concentration, M tg (sec) Initial Final 0.025M Green Gray-M Green Gray-M Green Gray-M Green Gray 442 Regression analysis using above mentioned equations was summarized on Table 3.5. The order with R closest to one dictates the reaction order. Thus, the order of the reaction was experimentally determined to be zero order with an R2 value equal to 0.9959. 2 Table 3.5. Determination of the order of reaction from hydrolysis of trans-[CoCl 2(en) 2]+. Order of Reaction R2 slope y-intercept -4 Zero Order- -x- First Order- -x- -1 Second Order-x10 - The last part of the exercise dealt with the study of the temperature dependence of the hydrolysis of the reaction. The results of the experiment were summarized on Table 3.6 for the increasing temperature mode and Table 3.7 for the decreasing temperature mode. Table 3.6. Plotted data for the determination of the Arrhenius frequency factor and the Activation Energy for the increasing temperature mode with respect to the theoretical first order. Temperature, C 1/T, (K-1) t, sec k ln k -3 -x-x10 -x-x10-3 - -3 -x-x10 -x-x10-3 - -3 -x-x10 -x-x10-3 - Table 3.7. Plotted data for the determination of the Arrhenius frequency factor and the Activation Energy for the decreasing temperature mode with respect to the theoretical first order. Temperature, C 1/T, (K-1) t, sec k ln k -3 -x-x10 -x-x10-3 - -3 -x-x10 -x-x10-3 - -3 -x-x10 -x-x10-4 - The rate constant, k, were determined and a plot of ln k vs. 1/T was made based from the Arrhenius equation. The activation energy as well as the Arrhenius frequency factor were determined using the two modes and the calculated data were summarized on Table 3.8. The calculated energy of activation was 71.81 kJ/mol and 49.48 kJ/mol while the frequency factor was 3.377x108 sec-1 and 1.076x105 sec-1 for increasing and decreasing temperature mode, respectively. Table 3.8. Determination of the Arrhenius frequency factor and activation energy with respect to the theoretical first order reaction using the calculated parameters in increasing and decreasing temperature mode. Value Parameter Increasing Mode Decreasing Mode Slope - - y-intercept- R- Energy of Activation (kJ/mol- -1 Arrhenius frequency factor (sec - IV. Summary, Conclusion and Recommendation In this exercise, trans-[Co(en) 2Cl 2]Cl was synthesized through acid hydrolysis, the kinetics of acid hydrolysis was then studied and the order of the reaction and its dependence on the temperature was determined. The synthesized green crystals of trans-[Co(en) 2Cl 2]Cl was about 0.5263g which corresponds to 21.94% yield. A low percent yield can be accounted due to losses during transfer, not enough reaction time, reaction conditions not followed properly and other random error during the experiment. In the kinetic study and temperature dependence, the computed values for the Rg are 0.5814, 0.5769 ang 0.6098 with an average value of 0.5894. The calculated energy of activation was 71.81 kJ/mol and 49.48 kJ/mol while the frequency factor was 3.377x108 sec-1 and 1.076x105 sec-1 for increasing and decreasing temperature mode, respectively. The order of the reaction was also determined. Based from the experiment, the reaction was zero order. On the other hand, the theoretical order was first order since it is typical for an octahedral complex to have a first order mechanism. The rate of the reaction also depends not only on the geometry of the complex but also on the charge on the central metal ion, changes in the concentration of the incoming ligand, nucleophilicity of the incoming ligand, characteristic of leaving group and chelation. It is recommended to use different complexes in the study of the kinetics of reaction. Square planar complexes can also be studied to showcase the other mechanism. Careful execution of the procedure should also be practiced to have a greater yield of product. V. References 1. Cotton, F.A. and G. Wilkinson. 1988. Advanced Inorganic Chemistry. USA: John Wiley and Sons. Pp. 654 - 665 2. Housecroft, C.E. and A.G. Sharpe. 2005. An Introduction to Inorganic Chemistry. United Kingdom: Pearson Education Limited. Pp-. Miessler G.L. and Tarr D.A. Inorganic Chemistry. 3rd edition. Pearson Ed. Pp. 415-430