Werner Complexes - Preparation and Determination of Structural Formula

        The second technique for determining the amount of ions present in a Werner complex is magnetic susceptibility. With finding the magnetic susceptibility using a magnetic susceptibility of a compound, one can calculate whether the compound is paramagnetic or diamagnetic. If the mass magnetic susceptibility, χg, is negative, then the compound is diamagnetic; however, if χg is positive, then the compound is paramagnetic. The apparatus used for this experiment is a Evans/Johnson Mathey Magnetic Susceptibility Balance.

        The last technique used was titrating a sample with silver nitrate. Since the three samples contain chloride ions, titrating it with silver nitrate would precipitate silver chloride, which is very insoluble. Based on the volume of silver nitrate used to reach the end point, one can calculate the amount of silver chloride precipitated. The amount of silver chloride precipitated is also the amount of free chloride ions in the mixture. Comparing the ratio of moles of silver nitrate used to the moles of analyte in the mixture will show the amount of non-bonding chlorine present.

Procedure and observations:

Synthesis of hexaamminecobalt(III) chloride:

        The synthesis was started by adding cobalt(II) chloride hexahydrate(4.690g) and ammonium chloride (2.946g) to water(5mL) which formed a purple solution from the original red/pink powder. This solution was heated up and turned dark blue once it was all dissolved. Activating charcoal was then added as a catalyst - the solution was then black with a slight blue tint. Afterwards, concentrated ammonia (9.8mL) was added to the mixture. This created a very dark red solution, but upon setting, the color eventually faded to yellow with a bit of red in it. With this, the solution was then cooled in a tab water bath and hydrogen peroxide (10.0mL, 6%) was added and heated to 65 degrees Celsius for 17 minutes. To crystallize, the solution was then places in another tap water bath, proceeded by an ice water bath. The brown crystals were then filtered by vacuum filtration and transferred to another beaker containing concentrated hydrochloric acid (2.1mL) and water (41.0mL). After all the crystals were dissolved, they were filtered by gravity filtration and washed with water (1.7mL). The red/gold filtrate was collected, and another portion of concentrated hydrochloric acid(5.2mL) was added. The filtrate was then cooled on ice to crystallize once more. Upon cooling, the solution turned bright orange. The sides of the erlenmeyer flask had to be scratched vigorously to help induce the production of crystals. Finally, the crystals were filtered once more by vacuum filtration, and dried between two pieces of large filter paper. The final product were bright orange crystals(0.123g) at a 2.36% yield.

Synthesis of pentaamminechlorocobalt(III) chloride:

        To start the synthesis, solid ammonium chloride (7.560g) was added to a beaker containing ammonia (44.6mL, 14M). While stirring, finely powdered cobalt(II) chloride hexahydrate (7.465g) was slowly added. The solution turned orange/brown when approximately 1.8g was added. After complete addition the solution was a bright orange and pink color with a small brown layer on top. Hydrogen peroxide (7.2mL, 30%) was added to this mixture which created vigorous effervescence of oxygen gas and a very dark red color. Once the effervescence subsided, concentrated hydrochloric acid (46.1mL) was added - which created a light red color and blue gas on top with temporary bubbling. The solution was then heated to 85 degrees Celsius for 20 minutes, then cooled to room temperature resulting in a dual layered solution - pink on top, blue on the bottom. Afterwards, the pink precipitated layer was filtered by vacuum filtration. While filtering, the solid was washed with three portion of water (30.4mL total), followed by cold HCl (30.0mL, 6M) and ethanol (30.6mL, 100%). The product was then dried in an oven at 100 degrees Celsius for one hour. After drying, the product was a dull pink solid(6.421g) at a 81.7% yield.

Synthesis of hexaamminenickel(II) chloride:

        Nickel chloride (1.210g) was dissolved in ethanol (10.2mL, 95%) which produced a lime-green solution. Concentrated ammonia (5.1mL) was added to the solution, which produced a milky-purple color with a light purple precipitate. The precipitate was then vacuum filtered for 20 minutes and washed with ethanol (6mL, 95%). The product was light purple(0.951g) with 78.7% yield.

All three substances synthesized were analyzed to find conductance measurements, and magnetic susceptibility. The two cobalt products were titrated with silver nitrate to find the amount of free chloride ions contained in both.

Results and Data:

        Balanced Equations:

                                                                                                                Charcoal      

1) 2CoCl2*6H2O(s) + NH4Cl(s) + 10NH3(aq) + H2O2(aq) + 3H2O(l)       

2[Co(NH3)6]Cl3(s) + 1/2O2(g)

2) 2CoCl2*6H2O(s) + 2NH4Cl(s) + 8NH3(aq) + H2O2(aq) + 3H2O(l)       

2[Co(NH3)5Cl]Cl2(s) + 1/2O2(g)

3) NiCl2(s)   +   6NH3(s)    [Ni(NH3)6]Cl2(s)

4) Oxidation: Co3+ + e-      Co2+

5) Reduction: H2O2     H3O+ + e- + 1/2O2

        Theoretical Yields:

Hexaamminecobalt(III) chloride:

 *  =

Pentaamminechlorocobalt(III) chloride:

  =

Hexaamminenickel(II) chloride:

  =

        % Yields:

Hexaamminecobalt(III) chloride:

 * 100% = 2.38%

Pentaamminechlorocobalt(III) chloride:

 = 81.7%

Hexaamminenickel(II) chloride:

 = 78.7%

χg = Mass Susceptibility (in erg*G-2*mol-1)1

χA = True Molar Susceptibility (in erg*G-2*mol-1)1

μ = Magenetic Moment (in J/T)

S = Total spin for unpaired electrons

Sample calculation for [Co(NH3)6]Cl3:

C =  =  =

χg = [C(R-Ro)] = [1.0806*10-9(0.01-0)] = -2.401*10-10 erg*G-2*mol-1

χA = χM + χdia =  (-2.401*10-10 * 267.48g*mol-1) +1.91*10-4 = 0.000190936 erg*G-2*mol-1

μ = 2.828= 2.828= 0.672479301 J/T

S(S+1) =    = 0  S = 0.1300

0.1300 ~ 0 - therefore 0 unpaired electrons

Free chloride sample calculation for [Co(NH3)5Cl]Cl2 :

  =  mol

 =  mol

 = 1.82  2 equiv

Discussion:

        The results show that all three compounds are arranged to give the most stable geometries possible - octahedral. The three chloride ions in hexaamminecobalt(III) chloride, are not bonded to the cobalt metal because there are six amine ligands to form the octahedral. The same thing goes with hexaamminenickel(II) chloride and its two non bonded chlorine ions. Pentaamminechlorocobalt(III) chloride on the other hand only has five amine ligands in the lattice, so it pulls in a chlorine atom to help stabilize its octahedral structure.

        With the titration free chloride analysis in the cobalt (III) compounds, the ratios between the amount of analyte in the solution, and the amount of silver nitrate needed to reach the end point and precipitate all the chloride into silver chloride gave the amount of non bonding chloride ions. In the example shown in the results section, 4.44*10-4mol of silver nitrate was used to react with 1.99*10-4mol of pentaamminechlorocobalt(III) chloride. Since the ratio was nearly 2:1 silver nitrate to pentaamminechlorocobalt(III) chloride, this concludes that two moles of non bonded chlorine are present per one mole of pentaamminechlorocobalt(III) chloride. This portion of the structural analysis required the addition of 2% dextrin to each sample. Because the indicator (fluorescien) reacts on the surface of the silver chloride precipitate, dextrin was added to prevent coagulation and stabilize the colloid.

         From the analysis of the magnetic susceptibility portion of the experiment, as expected, the mass susceptibility of both cobalt(III) compounds was negative, indicating that they are both diamagnetic and have all paired electrons. In this case, the pentaamminechlorocobalt(III) chloride was -3.0823*10-6 erg*G-2*mol-1 and hexaamminecobalt(III) chloride was -2.401*10-10 erg*G-2*mol-1, which is proof for the low-spin state in Figure 1. The hexaamminenickel(II) chloride complex however was positive, 9.1438*10-6 erg*G-2*mol-1. Since this is a positive value, there must be some lone pairs of electron, to analyze how many there are, the total spin must be checked. The spin due to unpaired electrons in this case was ~.80, and for every 1/2 spin there is one unpaired electron.1 Rounding up, it can be concluded that hexaamminenickel(II) chloride has two unpaired electrons in its high spin state (see Figure 2).

 Figure 1: This figure shows the low spin state of hexaamminecobalt(III) chloride and pentaamminechlorocobalt(III) chloride, both with Cobalt3+ at its d6 electron configuration. Since the value of Δo is very large, the energy required for an electron to be placed in eg is alot higher than the energy required to pair the electrons in  t2g so the two compounds stay in the low-spin state.

Figure 2: This figure shows the high spin state of hexaamminenickel(II) chloride, with Nickel2+ at its d8 electron configuration. In this case, the magnitude of Δo does effect whether the compound is low spin or high spin since the two electrons in the eg have nowhere else to go. The low spin state of a d8 molecule is therefore rare since it would require additional energy to pair up the two eg electrons.

        Using a Conductivity Probe and a Vernier Interface Serial Box, the conductance of all three compounds was generated. A solution of approximately 10-3M of all three substances was analyzed to show the number of ions present in each solution. Measuring the conductance of each and comparing to the literature value shows the number of ions in one molecule. In pentaamminechlorocobalt(III) chloride, the conductance measured was 234.717ohm-1cm2mole-1, which corresponds to the literature value of  235 ohm-1cm2mole-1 which shows 3 total ions present in a molecule - one [Co(NH3)5Cl]2+, and two Cl- ions to balance the charge. The same can be seen with hexaamminecobalt(III) chloride, [Co(NH3)6]3+ + 3Cl- and hexaamminenickel(II) chloride [Ni(NH3)6]2+ + 2Cl-.

        There were may unusual reagents used during the synthesis of the two cobalt compounds. For hexaamminecobalt(III) chloride, activating charcoal was added. The carbon acts as a catalyst to speed up the reaction. The hydrogen peroxide (6%) was added in both of the cobalt compound synthesizes to act as an oxidizing agent for the Cobalt3+ to Cobalt2+. With the addition of hydrogen peroxide, vigorous effervescence occurred as it was being reduced and oxygen gas was evolving. During the final step of the synthesis of pentaamminechlorocobalt(III) chloride, ethanol was added. This is because ethanol is very volatile, so it greatly decreases the drying time of the final product.

        Even though all three compounds were produced successfully, only two of them had a decent yield. Pentaamminechlorocobalt(III) chloride and hexaamminenickel(II) chloride were produced at 81.7% and 78.7% respectively, whereas hexaamminecobalt(III) chloride was only produced at with a 2.38% yield. This was most likely due to inadequate suction while performing the various vacuum filtrations, which would have left some of the compound in the Buchner funnel when the product was in the filtrate.

Conclusion: 

        Through various experimental techniques the three coordination compounds, [Co(NH3)6]Cl3, [Co(NH3)5Cl]Cl2 and [Ni(NH3)6]Cl2 can easily be examined and characterized. The three methods were useful in characterizing a different part of each compound.

        Free chloride titrations can quickly produce the amount of non bonding chloride anions present in a compound’s crystal lattice. As shown in the structures above, hexaamminecobalt(III) chloride has three non bonding chloride ions, where pentaamminechlorocobalt(III) chloride and hexaamminenickel(II) chloride both have two non bonding chlorides.

        Conductance measurements can show the number of total ions available in a solution. Hexaamminecobalt(III) chloride has four total ions, one for the main structure, and three for the charge balancing chloride anions. Pentaamminechlorocobalt(III) chloride and hexaamminenickel(II) chloride both have three total ions, the main cation metal complex, and two chloride anions again to balance the charge

        Finally, Magnetic susceptibility is a simple way of determining that the compounds were diamagnetic or paramagnetic, and finding out how many lone pairs of electrons are included in the valence shell. Analyzing the results show the two cobalt based products have a low spin, diamagnetic electron arrangement, and the nickel complex has a high spin and paramagnetic electron arrangement.

References:

1) SFU Chem 236 Lab manual, pp24-25, Appendix III pp1-4, Appendix IV pp1 - 7.2) Coordination Compounds Help Page, http://www.chem.purdue.edu/gchelp/cchem (March 22, 2009).