Inorganic Chemistry COORDINATION COMPLEXES OF COBALT B. LINKAGE ISOMERISM IN [CO(III)(NH3)5NO2]2+

Purdue university Calumet Page 1 Werner Cmplx B
Inorganic Chemistry
COORDINATION COMPLEXES OF COBALT
B. LINKAGE ISOMERISM IN [CO(III)(NH3)5NO2]2+
Background
Isomers are compounds that have the same empirical formula but different three
dimensional structures. In transition-metal chemistry, different types of isomerism are
possible based on the number of ligands bonded to the metal, the arrangement of those
ligands, and/or the way in which the ligands bind to the metal ion. With the exception of
optical isomers, the physical properties of the different isomers are distinct and can be
used as the basis for differentiation and evaluating the structure of the complex.
The nitrite ion (NO2-) is an example of an ambidentate ligand. Ambidentate ligands can
coordinate to a metal center in more than one way and the nitrite ligand can coordinate to
a metal at either of two non-equivalent sites, from N or O. In this experiment, cobalt
complexes containing the NO2-ligand are investigated. Two different isomers, linkage
isomers, of the cobalt coordination complex are possible when nitrite is bound as a
unidentate ligand.
The two isomers will be prepared and data from the infrared spectra of the compounds
will be used to assign the two possible isomers to the proposed structures. The relative
stability of the two isomers will be predicted by computational methods. One isomer is
thermodynamically less stable than the other. In such cases, one can explain the
existence of the less-stable product (the kinetic product) as forming rapidly under the
initial conditions, and the more stable product (the thermodynamic product) forms as the
kinetic product undergoes further reaction. The kinetic product of a reaction can often be
isolated by a variety of methods, including rapid precipitation or low-temperature
synthesis.
Preparation of the Linkage Isomers of [Co(NH3)5(NO2)]Cl2
Isomer I
Dissolve 1.5 g of [Co(NH3)5Cl]Cl2 in an Erlenmeyer flask containing a solution of 25 mL
water and 5 mL concentrated aqueous ammonia. Warm the mixture (~80°C) on a hot
plate in the hood, stirring with a metal spatula, until the salt dissolves (it may be
necessary to boil to ensure complete dissolution of the starting material). Filter the
solution to remove any residual solids and cool the filtrate to room temperature. (Discard
the solid in an appropriate solid waste container.)
Add 6 M HCl dropwise to the filtrate until the solution is just neutral to universal
indicator paper (approximately 15 ml is needed). It is important that this solution not be
alkaline. Add 1.6 g of sodium nitrite (NaNO2) to the cooled solution followed by 1.6 ml
of 6 M HCl. Allow the reaction mixture to stand in an ice bath for at least an hour.
After the solution has cooled and the product has precipitated, filter the mixture through a
Buchner funnel and then wash the yellow-orange solid (Isomer I) with a small volume of
ice-cold water, followed by cold 95% ethanol and then diethyl ether. Allow the product
to dry. It is important to obtain the visible spectrum and IR spectrum of this product
immediately after synthesis.
Purdue university Calumet Page 2 Werner Cmplx B
Inorganic Chemistry
Isomer II
Dissolve 0.5 g of Isomer I, in 5.0 mL of hot water containing 1.0 mL of concentrated
aqueous ammonia. Cool the solution in an ice bath, and add 5.0 mL of concentrated HCl.
Allow the mixture to stand in an ice bath until precipitation is complete. Collect the
brown-orange crystalline product by filtration through a Buchner funnel, and wash the
pale solid with 25 mL of cold 95% ethanol. Allow the product to air-dry. Record the IR
spectrum and the visible spectrum of the product.
Molecular Modeling
This may be performed outside of normal lab hours. Build and minimize the energy of
the nitro and nitrite isomers of [Co(III)(NH3)5(NO2)]2+. (You may use ChemBio 3D and
minimize using molecular mechanics.) Which isomer is more stable? How does your
synthetic procedure lend itself to isolating the kinetic product?
Characterization of [Co(NH3)5ONO]Cl2, [Co(NH3)5NO2]Cl2, [Co(NH3)5Cl]Cl2 and
[Co(NH3)6]Cl3
Obtain a visible spectrum (400 to 650 nm) for dilute aqueous solutions of each of the four
complexes.
Collect IR spectra of the two isomers, [Co(NH3)5ONO]Cl2 and [Co(NH3)5NO2]Cl2, the
starting material, [Co(NH3)5Cl]Cl2, and the hexamine complex, [Co(NH3)6]Cl3. Also
record the IR spectrum of a sample of K3[Co(NO2)6].
References
1. Williams, G. M.; Olmsted, J., III; Breska, A. P., III. J.Chem. Ed.1989, 66, 1043.
2. Jolly, W. L. The Synthesis and Characterization of Inorganic Compounds;
Prentice Hall: Englewood Cliffs, NJ, 1970; p463.
3. Richens, D. T.; Glidewell, C. Linkage Isomerism: An Infra-Red Study. In
Inorganic Experiments, J. D. Woolins, Ed.; VCH Publisher, Inc.:New York, 1994,
pp. 26-27.
4. Preparation of some Werner Complexes,
wwwchem.uwimona.edu.jm:1104/lab_manuals/Werner2.html

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