Question

In Example \(24-5,\) we chose between a tetrahedral and a square-planar structure for \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}\) based on magnetic properties. Could we similarly use magnetic properties to establish whether the ammine complex of \(\mathrm{Ni}(\mathrm{II})\) is octahedral \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) or tetrahedral \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} ?\) Explain.

Short answer

No, it's not possible to use magnetic properties to determine if the ammine complex of Ni(II) is octahedral or tetrahedral because Ni(II) most likely forms a diamagnetic, low-spin, octahedral complex with NH3 in both proposed configurations.

Step-by-step solution

Explain the concept of spin and crystal field theory.

According to crystal field theory, in high-spin octahedral complexes, electrons would rather occupy higher energy levels than pair up in lower energy levels - it's easier for them to overcome the relatively small energy difference than to pair up and cause high amounts of repulsion. This results in high-spin complexes displaying paramagnetism, as unpaired electrons cause a magnetic field. Contrarily, in low-spin complexes, the energy difference between the levels is too great and electrons are forced to pair up in lower energy levels, resulting in diamagnetism, as there are no unpaired electrons to create a magnetic field.

Discuss Ni(II) and its expected behavior.

Ni(II) has an electron configuration of [Ar] 3d8 4s2. The Ni(II) ion is created by losing the two 4s electrons, hence has an electron configuration of [Ar] 3d8. Given the low charge of the ligand, NH3, in comparison to other ligands (like CN-), we would expect an octahedral arrangement to occur which in the case of Ni(II) would result in a diamagnetic complex as the energy difference between the t2g and eg orbitals is too great, and it's preferable for the two electrons to pair up in the t2g orbitals.

Discuss the possibility of a tetrahedral arrangement.

While recognizing that a low-spin tetrahedral Ni(II) complex is unlikely, it is important to discuss why. Tetrahedral complexes do not force electrons to couple as octahedral complexes do, resulting in unpaired electrons and paramagnetism. But since NH3 is a weak field ligand, and Ni(II) tends to form six coordinate complexes, the formation of a tetrahedral complex is improbable.

Key concepts

Crystal Field Theory

Crystal Field Theory (CFT) is a model that describes how the arrangement of ligands around a central metal ion influences the energy of the d-orbitals of the metal. In coordination complexes, the spatial distribution of the ligands produces an electrostatic field that can split the energies of these d-orbitals. This difference in energy levels is termed the crystal field splitting.

The strength of this splitting varies depending on the geometric arrangement of the ligands and the type of ligands attached. It is crucial to note that the splitting pattern differs for octahedral, tetrahedral, and square-planar geometries. For example, in an octahedral geometry, the d-orbitals split into two sets: the lower-energy t2g orbitals and the higher-energy eg orbitals. Understanding CFT is vital to predicting properties like the magnetic character and color of the complex.

High-Spin vs Low-Spin Complexes

The concept of high-spin and low-spin complexes arises from the magnitude of crystal field splitting in coordination complexes. High-spin complexes occur when the energy required to pair up electrons in the same orbital is greater than the energy needed to place an electron in a higher energy orbital. This results in fewer electron pairs and more unpaired electrons, which is why high-spin complexes are usually paramagnetic.

On the other hand, low-spin complexes form when the crystal field splitting energy is higher. In this case, it's energetically more favorable for electrons to pair up in the lower energy orbitals before occupying the higher ones. This leads to a minimal number of unpaired electrons or none at all, leading to diamagnetic behavior. The spin state of a complex can greatly affect its reactivity and properties.

Tetrahedral and Octahedral Geometries

Tetrahedral and octahedral geometries are common shapes found in coordination complexes. Octahedral complexes are formed by six ligands symmetrically arranged around a central metal ion, while tetrahedral complexes have four ligands oriented towards the corners of a tetrahedron.

The crystal field splitting is different for these shapes. In an octahedral complex, the splitting is more substantial, often leading to low-spin configurations. Meanwhile, tetrahedral complexes have a smaller splitting due to the less symmetrical arrangement of the ligands, and hence they often result in high-spin configurations. The geometry not only influences the electronic structure but also plays a role in the reactivity and stability of the complexes.

Paramagnetism and Diamagnetism

Paramagnetism and diamagnetism are magnetic properties related to the electronic configurations of atoms or molecules. Paramagnetic substances have one or more unpaired electrons and are attracted to magnetic fields. These unpaired electrons contribute to a magnetic moment, causing the substance to behave as a weak magnet.

Diamagnetic substances, on the other hand, have no unpaired electrons. Each electron is paired with another electron with opposite spin, cancelling out their magnetic moments. As a result, diamagnetic substances are not attracted to magnetic fields; in fact, they are slightly repelled by them. Understanding these properties helps predict the behavior of coordination complexes in magnetic fields.

Ni(II) Electron Configuration

The electron configuration of a Ni(II) ion, which is [Ar] 3d8, is influential in determining the magnetic and geometric properties of its complexes. For instance, a Ni(II) ion in an octahedral complex with a strong-field ligand could potentially adopt a low-spin configuration by pairing all its electrons in the t2g orbitals, becoming diamagnetic.

However, in the presence of weaker field ligands, such as NH3, a high-spin octahedral complex or even a low-spin state is possible. Ni(II) complexes are versatile in behavior, displaying markedly different physical characteristics depending on their ligands and the resultant geometric arrangement.