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 NC

Short answer

The given complexes \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) and \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) have one and five unpaired electrons, respectively. The \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) complex is low spin since it has minimal unpaired electrons, whereas the \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) complex is high spin since it has the maximum possible unpaired electrons for iron(III). From these results, we can conclude that the \(\mathrm{NCS}^{-}\) ligand is on the left side of the spectrochemical series as it forms a high spin complex, meaning it is a weak-field ligand.

Step-by-step solution

Understanding high spin and low spin complexes

High spin and low spin complexes refer to the arrangement of electrons in the d-orbitals of a metal ion in a coordination complex. High spin complexes have the maximum number of unpaired electrons possible, whereas low spin complexes have the minimum number of unpaired electrons possible. This difference arises due to the varying strengths of the ligands to split the d-orbitals.

Determine if the given complexes are high spin or low spin

We are given that the \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) complex has one unpaired electron, and the \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) complex has five unpaired electrons. Iron(III) has a d5 high spin electron configuration which means that all the d-orbitals have single electrons and no orbital pairing occurs. Since the \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) complex has only one unpaired electron, we can conclude that the ligand \(\mathrm{CN}^{-}\) has resulted in a low spin complex. In contrast, the \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) complex has five unpaired electrons, which is the maximum possible for an iron(III) complex, giving us a high spin complex.

Determine the placement of \(\mathrm{NCS}^{-}\) in the spectrochemical series

The spectrochemical series is a list of ligands ordered according to their ability to split the d-orbitals of the central metal ion. Ligands on the left side of the series are weak-field ligands and result in high spin complexes, while ligands on the right side are strong-field ligands and result in low spin complexes. Since the \(\mathrm{NCS}^{-}\) ligand forms a high spin complex with iron(III) as shown in the \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) complex, we can conclude that the \(\mathrm{NCS}^{-}\) ligand is on the left side of the spectrochemical series (i.e., it is a weak-field ligand).

Key concepts

Coordination Complexes

Coordination complexes, sometimes simply called complexes, are compounds that consist of a central metal atom or ion bonded to multiple molecules or ions, known as ligands. The ligands have lone pairs of electrons that they donate to the metal atom, creating a coordination bond. These complexes can have a variety of shapes, colors, and chemical reactivity, depending on the metal and ligands.

Most commonly studied within inorganic chemistry, coordination complexes play a critical role in many biological processes and industrial applications. Examples include hemoglobin, which contains iron coordination complexes, and catalysts used in processes to manufacture important chemicals.

Ligand Field Theory

Ligand Field Theory (LFT) is a cornerstone of inorganic chemistry that describes how ligands affect the distribution of electrons in the d-orbitals of transition metal ions within coordination complexes. According to LFT, when ligands approach the central metal ion, they interact with its d-orbitals, which creates an energy difference called crystal field splitting. This concept is crucial to understand the magnetic properties, color, and reactivity of complexes.

Depending on the relative energies of the split d-orbitals, electrons may be arranged in different ways, resulting in high spin or low spin configurations. Large field splits will typically lead to low spin configurations, where electrons pair up in the lower energy orbitals, while small field splits favor high spin configurations with more unpaired electrons.

Spectrochemical Series

The spectrochemical series is a ranking of ligands based on their ability to split the d-orbitals of a central metal ion. Ligands that cause a large splitting are called strong-field ligands and are typically found on the right side of the series. These strong-field ligands often lead to low spin configurations. On the other hand, weak-field ligands, located on the left side of the series, cause less splitting and typically result in high spin complexes with more unpaired electrons.

Understanding where a ligand falls within the spectrochemical series can help predict whether a coordination complex will be high spin or low spin. This classification is fundamental when assessing the magnetic properties or certain aspects of the reactivity of the complex.

Unpaired Electrons

In coordination chemistry, the number of unpaired electrons in a complex is a key factor determining its magnetic properties. Unpaired electrons contribute to magnetic moments, leading to paramagnetism, where the substance is attracted to magnetic fields. Conversely, substances with all paired electrons exhibit diamagnetism and are slightly repelled by magnetic fields.

To determine if a metal complex has unpaired electrons, one needs to consider the electronic structure of the metal ion as well as the field strength of the ligands, as explained by ligand field theory. This concept ties directly into the high spin and low spin classifications, with high spin complexes having more unpaired electrons and thus stronger magnetic properties.