Question

Explain the following: (a) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) is paramagnetic and coloured. (b) \(\left(\mathrm{Ni}(\mathrm{CN})_{4}\right)^{2-}\) is square planar, but \(\left[\mathrm{Ni}(\mathrm{Cl})_{4}\right]^{2-}\) is tetrahedral. (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) is more stable than \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\), while \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) is more stable than \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) (d) Octahedral complexes are less stable than the square planar complexes. (e) \(\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}\) is a low-spin complex, but \(\left[\mathrm{CoF}_{6}\right]^{3-}\) is high-spin complex.

Short answer

a) The \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) complex is paramagnetic and coloured due to 5 unpaired electrons causing d-d transitions. b) \(\left(\mathrm{Ni}(\mathrm{CN})_{4}\right)^{2-}\) is square planar because CN- is a strong field ligand while \(\left[\mathrm{Ni}(\mathrm{Cl})_{4}\right]^{2-}\) is tetrahedral because Cl- is a weaker field ligand. c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) is more stable than \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) due to NH3 being a strong field ligand, and \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) is more stable than \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) due to water being a weak field ligand. d) In general, square planar complexes are more stable due to greater crystal field splitting. e) \(\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}\) is low-spin due to strong field ligand CN- and \(\left[\mathrm{CoF}_{6}\right]^{3-}\) is high-spin due to weak field ligand F-.

Step-by-step solution

Explain Paramagnetism and Color

The \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) complex is paramagnetic because the Fe3+ ion has 5 unpaired electrons (electron configuration of Fe3+ is \(3d^5\) in an octahedral environment), which accounts for its paramagnetism. Unpaired electrons also are responsible for color in transition metal complexes. The color arises from d-d electron transitions, where an electron jumps from one d-orbital to another absorbing light of a specific wavelength (color).

Explain Geometry Differences

The difference in geometry \(\left(\mathrm{Ni}(\mathrm{CN})_{4}\right)^{2-}\) (square planar) versus \(\left[\mathrm{Ni}(\mathrm{Cl})_{4}\right]^{2-}\) (tetrahedral) can be explained using the concept of the spectrochemical series. The CN- ion is a strong field ligand that induces a large crystal field splitting and allows the Ni2+ ion (with an electron configuration of \(3d^8\)) to pair up its electrons into the lower energy level d-orbitals forming a square planar complex. On the other hand, Cl- is a weak field ligand that causes less crystal field splitting and because the energy difference is smaller, it's easier for electrons to occupy the higher energy level d-orbitals - thus, forming a tetrahedral complex.

Explain Stability Differences

Stability of \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) vs \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) and \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) vs \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) can be explained considering the nature of ligands and charge of central metal ion. NH3 is a stronger field ligand with a greater ability to cause crystal field splitting and stabilize higher oxidation states. So, \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) is more stable. H2O is a weak field ligand and it better stabilizes the lower oxidation state, hence \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) is more stable.

Compare Octahedral & Square Planar Stability

In general, square planar complexes are considered more stable due to greater crystal field splitting induced by the strong field ligands usually involved in square planar complexes. Higher crystal field splitting leads to stronger metal-ligand bonds, enhancing stability. However, this is a general statement and can vary depending on the metal ion and ligands used.

Explain Spin States

Spin states of \(\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}\) (low-spin) and \(\left[\mathrm{CoF}_{6}\right]^{3-}\) (high-spin) can be attributed to the strength of ligands in the spectrochemical series. CN- is a strong field ligand and causes large crystal field splitting enabling pairing of electrons i.e., low spin state. F-, on the other hand, is weaker field ligand causing smaller crystal field splitting leading to a high spin state where electrons occupy all five d-orbitals singly before pairing.

Key concepts

Paramagnetism in Coordination Compounds

Paramagnetism occurs in coordination compounds when there are unpaired electrons in the d-orbitals of the central transition metal ion. These unpaired electrons align with magnetic fields, making the compound attracted to magnets. In the Fe₃₊(H₂O)₆ complex, the iron ion has five unpaired electrons (electron configuration 3d⁵), contributing to its paramagnetic behavior. Such compounds also tend to be colored, as the unpaired electrons can be excited to higher energy d-orbitals by absorbing certain wavelengths of light, which we perceive as color. Understanding paramagnetism helps us in studying and predicting the magnetic properties and electronic structures of coordination compounds.

Paramagnetism is a key indicator of the electronic arrangement within a complex and is used in various applications such as magnetic resonance imaging (MRI) and in determining the structure of molecules using spectroscopy.

Crystal Field Theory

Crystal Field Theory (CFT) is a model that explains the break in degeneracy of the metal ion's d-orbitals in a coordination compound due to the repulsion between the electrons in the d-orbitals and the electric field created by surrounding ligands. The theory illustrates why some complexes are colored and how the metal-ligand bond forms. It provides a way to understand the electronic structure and properties of metal complexes such as their magnetism, the arrangement of ligands, and the potential geometries of the complexes.

CFT helps in explaining why Ni(CN)₄^2- adopts a square planar structure. The cyanide ligand creates a strong crystal field that allows for the pairing of electrons in lower energy d-orbitals, leading to this preferred geometry.

Ligand Field Strength

Ligand field strength is crucial in determining the structure and properties of coordination compounds. The strength is often ranked using the spectrochemical series, which lists ligands from low to high field strength, indicating their ability to split the d-orbital energies of the central metal ion. Strong field ligands, like CN^-, induce greater splitting which can lead to low-spin configurations and influence the geometry of the complex. On the other hand, weak field ligands like Cl^-, cause less splitting resulting in high-spin complexes and possibly different geometries. The field strength of the ligands impacts the overall stability and reactivity of the coordination complex.

Geometrical Isomerism in Coordination Compounds

Geometrical isomerism occurs in coordination compounds when there is more than one possible arrangement of ligands around the central metal ion, creating compounds with distinct physical and chemical properties. This type of isomerism is particularly prevalent in complexes with coordination numbers 4 and 6, resulting in square planar, tetrahedral, or octahedral isomers. For instance, the different geometries of Ni(CN)₄^2- and Ni(Cl)₄^2- showcase how ligand field strength can influence the spatial arrangement within coordination complexes. Geometrical isomerism is an important concept for understanding reactions in coordination chemistry, as well as the functionality of catalysts and in biological systems.

Spin States in Coordination Compounds

Spin states refer to the arrangement of electrons in d-orbitals of the metal ion in a coordination compound, particularly whether electrons are paired or unpaired. This can result in high-spin or low-spin states, determined by the field strength of the ligands according to CFT. High-spin states occur when weaker field ligands are present, causing less splitting of the d-orbitals and allowing electrons to occupy all orbitals singly. In contrast, strong field ligands create a large splitting, leading to low-spin states with more paired electrons. The spin state affects the magnetic properties and can influence the reactivity of coordination compounds. The high-spin and low-spin states of different Co^3- complexes provide a perfect example of how ligand field strength influences the distribution of d-electrons and the resulting magnetic properties.

Stability of Coordination Compounds

The stability of coordination compounds depends on a variety of factors including the charge on the central ion, the nature of the ligands, and the overall geometry of the complex. As seen with [Co(NH₃)₆]³⁺, strong field ligands like NH₃ can stabilize higher oxidation states of a metal, leading to more stable complexes. Conversely, the stability of lower oxidation states is often favored by weak field ligands such as H₂O. The choice of metal ion and ligands, along with the structural arrangement, impacts the strength of the metal-ligand bond and thus, the stability of the compound. Understanding these factors is essential for the synthesis and application of coordination compounds in areas such as catalysis, material science, and medicine.