Question

The complex ion \(\mathrm{Fe}(\mathrm{CN})_{6}{ }^{3-}\) is paramagnetic with one unpaired electron. The complex ion \(\mathrm{Fe}(\mathrm{SCN})_{6}{ }^{3-}\) has five unpaired electrons. Where does \(\mathrm{SCN}^{-}\) lie in the spectrochemical series relative to \(\mathrm{CN}^{-}\) ?

Short answer

In the spectrochemical series, the complex ion \(\mathrm{Fe}(\mathrm{CN})_{6}{ }^{3-}\) has fewer unpaired electrons (one) than the complex ion \(\mathrm{Fe}(\mathrm{SCN})_{6}{ }^{3-}\) (five), indicating that \(\mathrm{CN}^{-}\) is a stronger ligand than \(\mathrm{SCN}^{-}\). Thus, \(\mathrm{SCN}^{-}\) lies lower in the spectrochemical series relative to \(\mathrm{CN}^{-}\).

Step-by-step solution

Identify the metal ion and its oxidation state in both complexes

We are given the complex ions \(\mathrm{Fe}(\mathrm{CN})_{6}{ }^{3-}\) and \(\mathrm{Fe}(\mathrm{SCN})_{6}{ }^{3-}\). The metal ion in both complexes is Iron (Fe). To find the oxidation state of the Fe ion in both complexes, we can set up the following equation: Fe + (oxidation state of ligand) * (number of ligands) = charge on complex For \(\mathrm{Fe}(\mathrm{CN})_{6}{ }^{3-}\), the oxidation state of \(\mathrm{CN}^{-}\) is -1, so we have: Fe + (-1) * 6 = -3 => Fe = +3 For \(\mathrm{Fe}(\mathrm{SCN})_{6}{ }^{3-}\), the oxidation state of \(\mathrm{SCN}^{-}\) is -1, so we have: Fe + (-1) * 6 = -3 => Fe = +3 In both complexes, the oxidation state of Fe is +3.

Analyze the unpaired electrons in each complex

We are given that \(\mathrm{Fe}(\mathrm{CN})_{6}{ }^{3-}\) has one unpaired electron and \(\mathrm{Fe}(\mathrm{SCN})_{6}{ }^{3-}\) has five unpaired electrons.

Determine the position of \(\mathrm{SCN}^{-}\) relative to \(\mathrm{CN}^{-}\) in the spectrochemical series

The unpaired electrons are determined by the energy difference between the paired and unpaired d-orbitals in a metal complex, which is affected by the ligand strength in the spectrochemical series. We are given that the complex ion with \(\mathrm{CN}^{-}\) has fewer unpaired electrons (one) than the complex ion with \(\mathrm{SCN}^{-}\) (five). This suggests that \(\mathrm{CN}^{-}\) is a stronger ligand than \(\mathrm{SCN}^{-}\) and causes a greater energy splitting between the d-orbitals. Therefore, \(\mathrm{SCN}^{-}\) lies lower in the spectrochemical series relative to \(\mathrm{CN}^{-}\), as it is a weaker ligand.