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?

In transition metal complexes, electrons are distributed between bonding and antibonding molecular orbitals. The energy difference between these orbitals, known as the crystal field splitting energy (\( \Delta\)), determines whether a complex is high spin or low spin. Weak field ligands result in a small \( \Delta\) and high spin complexes, while strong field ligands result in a large \( \Delta\) and low spin complexes. High spin complexes have more unpaired electrons, as electrons prefer to occupy degenerate orbitals before pairing up in lower-energy orbitals. Low spin complexes have fewer unpaired electrons, as they occupy lower-energy orbitals before moving to degenerate orbitals.

The complex \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) has one unpaired electron, while the complex \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) has five unpaired electrons. The larger number of unpaired electrons in the second complex indicates that it has a smaller \( \Delta\), making it a high spin complex. Conversely, the smaller number of unpaired electrons in the first complex indicates that it has a larger \( \Delta\), making it a low spin complex.

The spectrochemical series is the order of ligands based on their ligand field strength. The stronger the ligand, the larger the \( \Delta\), and the fewer the unpaired electrons (low spin complex). In this case, the CN- ligand forms a low spin complex, while the NCS- ligand forms a high spin complex. This indicates that the NCS- ligand has a weaker field strength than the CN- ligand. Thus, we can conclude that the NCS- ligand is placed lower in the spectrochemical series relative to the CN- ligand. In summary, the \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) complex is a low spin complex, while the \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) complex is a high spin complex. The NCS- ligand is placed lower in the spectrochemical series as compared to the CN- ligand, as it results in a high spin complex with a smaller \( \Delta\).