Synthesis and Isomerization of Nitro and Nitritopentaamminecobalt(III) Chloride, Linkage isomers.

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Kyle Thomas

April 1st 2009

Synthesis and Isomerization of Nitro and Nitritopentaamminecobalt(III) Chloride, Linkage isomers.

Abstract:

        In this experiment two compounds were synthesized, nitropentaamminecobalt(III) chloride and nitritopentaamminecobalt(III) chloride, which are a pair of linkage isomers. The infrared spectra of both compounds were measured to compare, and differentiate the two. Upon inspection, the nitritopentaamminecobalt(III) chloride compound had an asymmetric stretch at 1452cm-1 indicating a Co-ONO bond. Nitropentaamminecobalt(III) chloride does not have this peak, instead it has stretch at 500 - 435 cm-1 indicating a M-N bond, an asymmetric NO2 stretch at 1428cm-1 and a NO2 bend at 595cm-1. In both complexes, there’s a presence of ammonia degenerate, symmetric, asymmetric and rocking vibration frequencies.

        The infrared spectra of a sample of nitropentaamminecobalt(III) chloride heated in an oven overnight at 110 degrees celcius and pentaamminechlorocobalt(III) chloride were also measured for comparison to the two synthesized products. The heated nitropentaamminecobalt(III) chloride showed identical results to the non heated version, except the peak intensities were slightly less. The peaks on pentaamminechlorocobalt(III) chloride were all present on the infrared spectrum of the two synthesized products.

Introduction:

        The bent nitrite ion (NO2-) is an example of an ambidentate ligand, meaning it can form a bond from either the N, or O site. Two coordination compounds that can be bound at either of the two sites are called linkage isomers.  Even though the product would be in equilibrium, the N site is favored over the O site (see figure 1 and figure 2).

When prepared from pentaamminechlorocobalt(III) chloride the isomer obtained depends on the pH of the solution. If the solution is acidic, the nitrito complex will form, whereas the nitro complex will be produced in a basic environment.  Though the nitro isomer is more stable, it would take several months for the isomerization to occur, or can be greatly accelerated by heating.

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Figure 1 - The nitropentaamminecobalt(III) chloride complex is arranged in a low spin state with octahedral geometry. It’s composed of five ammine ligands and one nitro ligand attached to a cobalt center, with two chlorine counter ions to balance to charge. This compound is a yellow-brown color in solution or in solid state.

Figure 2 - Like the nitro isomer, the nitritopentaamminecobalt(III) chloride complex’s electrons are arranged low spin, and the ligands in an octahedral geometry. The instead of binding from the lone pair available on the nitrogen, in this state the cobalt is interacting with one of ...

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