Based on IR spectra of [Co(ONO)(NH3)5]Cl2 and [Co(NO2)(NH3)5]Cl2 complexes, indicate the differences.
The Correct Answer and Explanation is:
The IR spectra of the two cobalt complexes, [Co(ONO)(NH₃)₅]Cl₂ and [Co(NO₂)(NH₃)₅]Cl₂, can be used to distinguish between the bonding modes of the ligands, ONO⁻ (nitrito) and NO₂⁻ (nitro), which are attached to the cobalt center.
Key Differences in IR Spectra:
- Nitrito vs Nitro Ligand:
- In [Co(ONO)(NH₃)₅]Cl₂, the ONO⁻ ligand typically binds to the metal via the oxygen atom. This leads to an IR absorption in the range of ∼1500–1550 cm⁻¹ for the ν(O–Co) stretching vibration, characteristic of the nitrito group when it coordinates through the oxygen atom.
- In [Co(NO₂)(NH₃)₅]Cl₂, the NO₂⁻ ligand usually coordinates through the nitrogen atom. The ν(N–Co) stretching vibration of this nitro group appears in a different range, typically around ∼1300–1380 cm⁻¹.
- Symmetry of Ligands:
- ONO⁻ can exhibit different vibrational modes based on its binding, and the ν(O–N) stretching vibration often appears around ∼1260–1300 cm⁻¹.
- For NO₂⁻, the ν(N–O) stretching vibration typically appears at ∼1320–1370 cm⁻¹ for the asymmetric stretching mode, and the ν(N–O) symmetric stretch might appear slightly lower.
- Ligand-Specific Bands:
- NO₂⁻ has a characteristic asymmetric stretching band that appears in the range of ∼1500–1600 cm⁻¹ (depending on the exact environment), whereas ONO⁻ does not have such a prominent band in this region.
- The NO₂ complex will also display a ν(NO₂) stretching band around 1350 cm⁻¹, indicative of the NO₂⁻ group, which is not present in the spectrum of the ONO⁻ complex.
Summary:
The key difference between the IR spectra lies in the distinct stretching frequencies of the NO₂ and ONO ligands. The [Co(ONO)(NH₃)₅]Cl₂ complex exhibits a higher-frequency ν(O–Co) band and a characteristic ν(O–N) band, while [Co(NO₂)(NH₃)₅]Cl₂ displays distinct ν(N–O) stretching vibrations and a prominent ν(NO₂) band. These spectral features help differentiate between the nitrito and nitro coordination modes.
