Question

The ion \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) has one unpaired electron, whereas \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) has five unpaired electrons. From these results, what can you conclude about whether each complex is high spin or low spin? What can you say about the placement of \(\mathrm{NCS}^{-}\) in the spectrochemical series?

Short answer

Based on the provided information, \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) is a low spin complex with a strong-field ligand, while \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) is a high spin complex with a weak-field ligand. This indicates that the \(\mathrm{NCS}^{-}\) ligand is placed lower in the spectrochemical series compared to the \(\mathrm{CN}^{-}\) ligand, as weak-field ligands appear lower in the series.

Step-by-step solution

Understand high spin and low spin

High spin and low spin complexes refer to the distribution of electrons within the d-orbitals of a metal ion in a coordination complex. In a high spin complex, the electrons tend to occupy different orbitals with the same spin (maximizing unpaired electrons), while in a low spin complex, the electrons preferentially pair up within the orbitals, minimizing the number of unpaired electrons. The type of complex depends on the strength of the ligand field generated by the ligands surrounding the metal ion. High spin complexes form with weak-field ligands, while low spin complexes form with strong-field ligands.

Analyze the unpaired electrons in the complexes

We are given that \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) has one unpaired electron and \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) has five unpaired electrons. Considering that the \(\mathrm{Fe}\) ions in the complexes have the same oxidation state, we can compare the number of unpaired electrons to determine the spin state of the complexes.

Determine the spin state of each complex

As \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) has only one unpaired electron, it is a low spin complex, indicating that the \(\mathrm{CN}^{-}\) ligand is a strong-field ligand. On the other hand, \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) has five unpaired electrons, which indicates that it is a high spin complex, suggesting that the \(\mathrm{NCS}^{-}\) ligand is a weak-field ligand.

Make conclusions about the \(\mathrm{NCS}^{-}\) ligand in the spectrochemical series

Since \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) is a high spin complex and \(\mathrm{NCS}^{-}\) is a weak-field ligand, we can conclude that \(\mathrm{NCS}^{-}\) is placed lower in the spectrochemical series as compared to the \(\mathrm{CN}^{-}\) ligand. The spectrochemical series is a ranking of ligands based on the strength of their ligand field; hence, weak-field ligands appear lower in the series.