Lab 1. Preparation of Hexaamminecobalt (III) Chloride

Lab 1. Preparation of Hexaamminecobalt (III) Chloride

Introduction

This experiment illustrates the preparation of a coordination compound of cobalt. You will determine whether or not the chlorine atom is coordinated to the cobalt.

The 6-coordinate cobalt(III) compound is prepared from CoCl2⋅6H2O. This is made difficult by the fact that Co2+ ion is more stable than Co3+ forsimple compounds. There are only a few simple compounds of cobalt(III) known, such as CoF3. However,cobalt(III) can be stabilized in octahedrally coordinated compounds.

To determine if the chlorine atom is coordinated or ionic, a gravimetric analysis of the chloride could be used to precipitatewith silver ions.An easier method involves conductivity measurements to determine the number of ions in solution.

A difficulty in this experiment is the oxidation of cobalt(II) to cobalt(III). This can beaccomplished by air oxidation of cobalt with carbon as acatalyst. An additional benefit of carbon as a catalyst is its ability to shift the equilibrium infavor of the desired product.

Fig.Lewis bases can approach and subsequently "coordinate" to transition metal cations via the participation of nonbonding electron pairs. Electron deficient transition metal cations are strong Lewis acids.

Experimental Procedure

Preparation of Hexaamminecobalt(lII) Chloride

Add 1.25g of CoCl2⋅6H2O and 0.82 g of NH4Cl to 8 mL of de-ionized water in a 25-mLErlenmeyer flask. Under the bench fumehood, add 0.25 g activated charcoal and 10mL conc. aqueous ammonia (smelly!).

Cool the brown slurry in an ice bath to 0°C, then add 2.0 mL (in a small graduated cylinder) 15% H2O2(caution) slowly using an eye dropper to transfer. Do notallow the temperature to rise above 10°C.

Heat the resulting red-brown solution to 60°C, and maintain this temperature for 20min. This heating is needed to ensure completedisplacement of all aquo ligands.

Cool the mixture to 0°C and the product will precipitate fromthe solution. Collect the product and the charcoal by vacuum filtration.

Recrystallization of the solid is necessary to separate the product from the activated charcoal as follows:

Transfer the solid to a 25-mL Erlenmeyerflask, and add 10 mL hot water and 0.25 mL conc. HCl (test the solution with litmus paper to make sure the solution is slightly acidic, adding a few more drops of HCl if needed).

Heat the mixture to 70°C, and filter while still hot using folded filter paper.

Place the filtrate in an ice bath, and add 0.25 mL cold conc. HCl. Collect the orange solid by vacuum filtration, wash with 5 mL ice-cold ethanol,and allow to air-dry.

Report

Q1. Weigh the mass of the dry product and calculate the % yield from the equation.

Store the product in a vial, provided by the instructor, and label.

Q2. Measure the conductivity of the compound. How many ions are present?

Q3. Record the IR spectrum of the compound in a Nujol mull between polyethylene discs, as well as between NaCl discs. What bands can you identify?

Q4. Record the UV-Vis spectrum of the compound (approx. 2 mg of complex in 50 mL deionized water). What does this tell you about the purity of the compound?

Q5. What can you tell about the structure of the compound from this data?