Linkage Isomers: Synthesis and Characterization of [Co(NH3)5ONO]Cl2 and [Co(NH3)5NO2]Cl2

Linkage Isomers: Synthesis and Characterization of

[Co(NH₃)₅ONO]Cl₂ and [Co(NH₃)₅NO₂]Cl₂

Divalent cobalt is more stable than trivalent cobalt for simple salts of cobalt, with only a

few salts of Co(III) known (e.g. CoF₃). Formation of a coordination complex stabilizes a

higher oxidation state as evidenced by a number of octahedrally coordinated complexes. In

this experiment, you will prepare1, 2 two classical linkage isomer compounds,

[Co(NH₃)₅ONO]Cl₂ and [Co(NH₃)₅NO₂]Cl₂.

1) One synthesis scheme of [Co(NH₃)₅Cl]Cl₂ involves the following unbalanced equations

Co²⁺ + NH⁴⁺ + 1/2H₂O₂ → [Co(NH₃)₅(OH₂)]³⁺

[Co(NH₃)₅(OH₂)]³⁺ + 3HCl → [Co(NH₃)₅Cl]Cl₂(s) + H2O + 3H⁺

2) Synthesis of [Co(NH₃)₅ONO]Cl₂ and [Co(NH₃)₅NO₂]Cl₂ involves the following:

[Co(NH₃)₅Cl]₂+ + H₂O → [Co(NH₃)₅H2O]³⁺ + Cl^-

[Co(NH₃)₅H₂O]³⁺ + NO^2- → [Co(NH₃)₅ONO]²⁺ + H₂O

[Co(NH₃)₅ONO]²⁺ → [Co(NH₃)₅NO₂]²⁺

Synthesis and Experimental Procedure:

All reactions should take place in a fume hood. Part I may be skipped if [Co(NH₃)₅Cl]Cl₂ is

available from a previous experiment. Scale solvent and reagent amounts as needed.

Part I: chloropentaamminecobalt(III) chloride, [Co(NH₃)₅Cl]Cl₂

Prepare a solution of 0.0935 mol ammonium chloride NH4Cl in 30 mL of concentrated

aqueous ammonia in a 250 mL Erlenmeyer flask. While stirring with magnetic stirbar,

slowly add 0.042 mol of cobalt(II) chloride hexahydrate. While continuously stirring,

slowly add 8 mL of a 30% H₂O₂ solution (CAUTION: H₂O₂ is a strong oxidizing agent

that can cause severe burns. Use gloves! Wash affected areas immediately with water!).

When evidence of further reaction has ceased, slowly add 30 mL of concentrated HCl. Heat

the solution using a hot plate. Maintain a temperature of about 85°C for about 20 minutes.

Do not allow the solution to boil. Cool the solution slightly and then place in an ice water

bath. Isolate the crystalline product by suction filtration. Wash the product with a small

amount of ice-cold water. The total volume of the wash should not exceed 15 mL. Wash

with 15 mL of cold 6M HCl. Dry in a drying oven for 20 minutes and then weigh the

product to determine the percent yield. Allow the sample to dry on the lab bench or drying

oven before collecting an IR and UV-Vis spectra. Discard the filtrate wash solution in the

appropriate “chloropentaamminecobalt(III) chloride” waste container in the fume hood.

Don’t tightly cap the waste bottle. 2

Part II: pentaamminenitritocobalt(III) chloride, [Co(NH3)5ONO]Cl2

Heat a solution (do not boil) of 8 mL of concentrated aqueous ammonia in 80 mL of water.

While heating and stirring, add 0.020 mol of ‘dry’ [Co(NH3)5Cl]Cl2 or 0.024 mol ‘wet’

[Co(NH3)5Cl]Cl2. Continue heating, below boiling point, and stirring until the product

dissolves. Add an additional small amount of 1:10 ammonia:water if needed. This may

take 15 minutes or more. If you observe the presence of a dark brown to black precipitate

of cobalt oxide, filter it off. Cool the solution, in an ice bath, to about 10 °C. Add 2 M HCl

slowly while keeping the solution cold until it is neutral as determined by litmus or pH

paper. Add 5.0 g of sodium nitrite followed by 5 mL of 6 M HCl. Cool the solution in an

ice bath for approximately an hour, then filter the precipitated salmon pink crystals of

[Co(NH3)5ONO]Cl2. Wash with 20 mL of icewater then 20 mL of 95% ethanol. Allow it

to dry on the lab bench for approximately one hour before collecting an IR and UV-Vis

spectrum. Do not dry in a drying oven. Discard the waste in the appropriate waste container

in the fume hood.

The product [Co(NH3)5ONO]Cl2 is not stable and will isomerize to [Co(NH3)5NO2]Cl2.

Ensure you have sufficient time in the laboratory session to collect the IR spectrum of the

product on the same day as the synthesis.

Pentaamminenitrocobalt(III) chloride, [Co(NH3)5NO2]Cl2

The product [Co(NH3)5ONO]Cl2 will isomerize in the solid state to [Co(NH3)5NO2]Cl2,

even at room temperature, given sufficient time.3

 Dr. Fred Basolo, winner of the Priestley

Medal (the highest honor conferred by the American Chemical Society), reported on the

kinetics and mechanisms of these isomerism reactions in the first paper4

 published by the

ACS journal Inorganic Chemistry.

Collect IR spectra over at least two laboratory periods. Compare the data over time and

show to the instructor.

Ensure UV-visible and IR spectra have been collected for each of the three products,

[Co(NH3)5Cl]Cl2, [Co(NH3)5ONO]Cl2, and [Co(NH3)5NO2]Cl2.

Retain your products in labeled sample bags.

If instructed, the isomerization may also be facilitated by heating. It is possible to use the

[Co(NH3)5ONO]Cl2 product before it is completely dry. Bring 20 mL of H2O to a boil and

add a few drops of NH4OH. Turn off heat and allow cooling for 4 minutes after boiling has

ceased. Add 2.0 g of [Co(NH3)5ONO]Cl2 to the solution. As this solution cools to room

temperature, add 20 mL of concentrated HCl. After cooling from solution,

[Co(NH3)5NO2]Cl2 will precipitate from solution. Filter the product from solution and wash

with small portions of a total of 10 mL 95% ethanol. Allow to dry under vacuum for

several minutes until dry and collect the IR spectrum and the UV-Vis spectrum. 3

References

1. Jolly, W. L. In The synthesis and characterization of inorganic compounds; PrenticeHall, Inc.: 1970;

2. Tanaka, J.; Suib, S. L. In Experimental methods in inorganic chemistry; Prentice-Hall,

Inc.: 1999;

3. R. B. Penland, T. J. Lane, J. V. Quagliano "Infrared Absorption Spectra of Inorganic

Coordination Complexes. VII. Structural Isomerism of Nitro- and

Nitritopentamminecobalt(III) Chlorides", J. Am. Chem. Soc. 78, 887 (1956).

4. F. Basolo, G. S. Hammaker "Synthesis and Isomerization of Nitritopentammine

Complexes of Rhodium(III), Iridium(III), and Platinum(IV)", Inorg. Chem. 1, 1 (1962).