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 SCN \(^{-}\) lie in the spectrochemical series relative to \(\mathrm{CN}^{-} ?\)

Short answer

In conclusion, SCN \(-\) lies below CN \(-\) in the spectrochemical series. This difference in their positions accounts for the difference in their paramagnetism and the unpaired electron count in the two given complex ions.

Step-by-step solution

Understand the Ligands in Complex Ions

In the given exercise, we have two complex ions: \(\mathrm{Fe}(\mathrm{CN})_{6}^{3-}\) and \(\mathrm{Fe}(\mathrm{SCN})_{6}^{3-}\). To understand the electronic configurations of these complex ions and their paramagnetism, we need to know about the ligands, which are the CN \(^{-}\) and SCN \(^{-}\) ions.

Recall the Spectrochemical Series

The spectrochemical series is an order of ligands based on the strength of their interaction with the central metal ion. Strong-field ligands result in a larger splitting of the d-orbitals, while weak-field ligands cause lesser splitting. A larger splitting leads to the pairing of electrons, reducing the number of unpaired electrons in a complex ion. Conversely, smaller splitting results in a higher number of unpaired electrons.

Observe the Paramagnetism of Complex Ions

From the given information, we have that \(\mathrm{Fe}(\mathrm{CN})_{6}^{3-}\) is paramagnetic with one unpaired electron, and \(\mathrm{Fe}(\mathrm{SCN})_{6}^{3-}\) is also paramagnetic but with five unpaired electrons. This information tells us that the CN \(^{-}\) ligand causes a large splitting in the d-orbitals, leading to a higher degree of electron pairing, while the SCN \(^{-}\) ligand results in a smaller splitting, producing a higher number of unpaired electrons.

Compare CN \(^{-}\) and SCN \(^{-}\) in the Spectrochemical Series

Based on the number of unpaired electrons and their effect on the splitting of d-orbitals, it is clear that CN \(^{-}\) is a stronger field ligand than SCN \(^{-}\). Thus, SCN \(^{-}\) lies lower in the spectrochemical series relative to CN \(^{-}\). In conclusion, SCN \(^{-}\) lies below CN \(^{-}\) in the spectrochemical series. This difference in their positions accounts for the difference in their paramagnetism and the unpaired electron count in the two given complex ions.

Key concepts

Complex Ions

Complex ions are fascinating structures consisting of a central metal atom or ion surrounded by molecules or ions known as ligands. They form when ligands donate electron pairs to the metal atom, creating a coordination bond. This bond is a unique interaction where non-bonded electrons pair up with empty orbitals on the metal.
For complex ions, the metal atom serves as the core, while the ligands encircle it. In the context of the given exercise, we were introduced to Fe(CN) ₆ ^3- and Fe(SCN) ₆ ^3- . Here, "Fe" represents the iron metal, and "CN^-" or "SCN^-" are the ligands.
Complex ions are very important in chemistry because they help us understand bonding interactions and properties such as color, magnetic behavior, and reactivity. By observing both the central metal's properties and the type and position of the ligands, we can determine the overall characteristics of the complex ion.

Paramagnetism

Paramagnetism is an intriguing property often present in materials or molecules with unpaired electrons. It involves the inclination of a material or ion to be drawn towards an external magnetic field.
In complex ions, the number of unpaired electrons plays a crucial role in determining whether a substance will exhibit paramagnetism. Take for example the two complex ions from the original exercise:

• Fe(CN) ₆ ^3-: This has one unpaired electron, leading to weaker paramagnetic behavior.
• Fe(SCN) ₆ ^3-: This contains five unpaired electrons, making it significantly more paramagnetic.

The difference in the number of unpaired electrons arises because of the disparity in how ligands affect d-orbital splitting of the central metal ion. As a general rule, more unpaired electrons lead to stronger paramagnetic properties. By understanding the arrangement of electrons, we better predict a complex ion’s magnetic nature.

Ligands

Ligands are integral to the formation and behavior of complex ions. They are ions or molecules able to donate electron pairs to the central metal ion, and this donation forms a coordinate covalent bond.
The spectrochemical series orders ligands based on their field strength — in other words, how well they can split the d-orbitals of the metal ion. Strong-field ligands cause greater splitting of these orbitals than weak-field ligands. Common strong-field ligands include CN^- (cyanide), while SCN^- (thiocyanate) is known as a weaker-field ligand.
The ability of a ligand to influence d-orbital splitting directly affects the electron configuration of the complex ion:

• Strong-field ligands: Lead to more electron pairing, resulting in fewer unpaired electrons.
• Weak-field ligands: Allow more unpaired electrons due to less splitting.

Understanding the role of ligands aids in predicting the structural and magnetic properties of complex ions. As explored in our exercise, the difference in ligand strength explains why Fe(CN) ₆ ^3- has fewer unpaired electrons than Fe(SCN) ₆ ^3- , highlighting CN^- as a stronger-field ligand than SCN^-.