Comparing IR Spectra: Analysis of NH2-, CO3^2-, and NO3- Absorptions in Different Cobalt Complexes

When analyzing the IR spectra of coordination complexes like [Co(NH3)4CO3]NO3 and [Co(NH3)5Cl]Cl2, it's important to consider the functional groups present and their characteristic absorption bands. Here's a breakdown of the expected absorptions and the similarities and differences between these two complexes: 1. **NH3 Ligands:** - Both complexes contain NH3 as ligands, which will show N–H stretching vibrations typically around 3300-3500 cm⁻¹. Additionally, bending vibrations can be found around 1600 cm⁻¹. The presence of NH3 in both complexes means that they will show similar N–H stretch and bending absorption bands. 2. **Carbonate (CO3) Group:** - In [Co(NH3)4CO3]NO3, the carbonate ion (CO3²⁻) exhibits characteristic absorptions. Key absorptions for CO3²⁻ include: - Asymmetric stretch near 1400-1500 cm⁻¹ - Symmetric stretch around 1060-1100 cm⁻¹ - Out-of-plane and in-plane bends around 700 cm⁻¹ and 880 cm⁻¹ - These absorptions are absent in [Co(NH3)5Cl]Cl2, which does not contain a carbonate ion. 3. **Chloride (Cl⁻) Ligands:** - While Cl⁻ is present in [Co(NH3)5Cl]Cl2, it does not give rise to prominent IR bands because Cl⁻ is an IR-inactive anion. Instead, the metal-chloride bonds may show weaker absorptions. 4. **Nitrate (NO3⁻) Ion:** - For [Co(NH3)4CO3]NO3, the NO3⁻ group will contribute characteristic absorptions: - Asymmetric stretch around 1380-1450 cm⁻¹ - Symmetric stretch near 1040-1080 cm⁻¹ - Bending vibrations can occur around 690-830 cm⁻¹ - These absorptions will be absent in [Co(NH3)5Cl]Cl2 since it does not contain NO3⁻. **Spectral Differences:** - [Co(NH3)4CO3]NO3 will exhibit additional absorption bands due to the CO3²⁻ and NO3⁻ ions that are not present in [Co(NH3)5Cl]Cl2. - Each complex contains different anions, which leads to the presence of distinct bands representing CO3²⁻ and NO3⁻ in [Co(NH3)4CO3]NO3, while [Co(NH3)5Cl]Cl2 might mainly show features attributable to NH3. **Spectral Similarities:** - Both complexes will show absorption bands due to the NH3 ligands, particularly in the N–H stretching and bending regions. In summary, evaluating the IR spectra involves analyzing the distinctive peaks from the carbonate and nitrate ions, as well as recognizing the N–H bands common to both complexes.

In IR spectroscopy, the technique essentially provides information about the types and strengths of bonds present in a molecule. Different bonding arrangements will give rise to different frequencies of vibration that can be observed on an IR spectrum as absorptions at different wavelengths. Firstly, it is important to understand the nature of the molecular entity we are dealing with in these two complexes: 1. [Co(NH3)4CO3]NO3: In this case, the complex consists of Co3+ coordinated to four NH3 molecules, one CO3 2- and a NO3- ion as outside—not part of a coordination complex. 2. [Co(NH3)5Cl]Cl2: In this case, the complex consists of Co3+ coordinated to five NH3 molecules and one Cl- ion, with 2 Cl- ions are outside of the coordination sphere. Now, let's analyze the characteristic absorptions of NH2-, CO3 2-, and NO3-. 1. NH3 groups: Both complexes contain NH3 so both should exhibit similar absorptions due to N-H stretching and bending vibrations. These typically occur in the range of about 3300-3500 cm-1 (stretching) and 1600-1650 cm-1 (bending). 2. CO3 2- and NO3-: The first complex contains a CO3 2- group within its coordination sphere. As such the CO3 2- characteristic absorptions (two peaks typically due to asymmetric and symmetric O=C=O stretching in the region of around 1400-1600 cm-1) should be observable. The second complex, however, does not contain a CO3 2- group and therefore would not show these absorptions. Conversely, the NO3- ion in the first complex is not part of the coordination sphere and should show characteristic NO3- absorptions (asymmetric stretching around 1380-1390 cm-1), while the second complex does not contain NO3-, and therefore would not show these absorptions. 3. Cl-: Only the second complex has Cl-. Chlorides generally absorb strongly at the far-infrared region, so you might not find characteristic absorptions in the normal IR spectrum range. Please remember this interpretation assumes there are no complex formation or solution effects that might shift or obscure these peaks, and we are referring to an ideal case. Actual spectra might vary due to numerous factors, including temperature, solvent effects, and sample purity.