Question

Nickel(II) complexes with the formula \(\mathrm{NiX}_{2} \mathrm{~L}_{2}\), where \(\mathrm{X}^{-}\) is \(\mathrm{Cl}\) or N-bonded \(\mathrm{NCS}^{-}\) and \(\mathrm{L}\) is the monodentate triphenylphosphine ligand \(\mathrm{P}\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{3}\), can be square planar or tetrahedral. (a) Draw crystal field energy-level diagrams for a square planar and a tetrahedral nickel(II) complex, and show the popula- tion of the orbitals. (b) If \(\mathrm{NiCl}_{2} \mathrm{~L}_{2}\) is paramagnetic and \(\mathrm{Ni}(\mathrm{NCS})_{2} \mathrm{~L}_{2}\) is diamagnetic, which of the two complexes is tetrahedral and which is square planar? (c) Draw possible structures for each of the \(\mathrm{NiX}_{2} \mathrm{~L}_{2}\) complexes, and tell which ones have a dipole moment.

Short answer

\(\mathrm{NiCl}_{2} \mathrm{~L}_{2}\) is tetrahedral and paramagnetic; \(\mathrm{Ni(NCS)}_{2} \mathrm{~L}_{2}\) is square planar and diamagnetic. Only \(\mathrm{NiCl}_{2} \mathrm{~L}_{2}\) may have a dipole moment.

Step-by-step solution

Understand Crystal Field Theory

Crystal Field Theory explains the electronic structure of transition metal complexes focusing on the electron occupancy of metal d orbitals. Depending on the energy level splitting, these complexes can adopt different geometries, such as square planar or tetrahedral.

Draw Orbital Splitting Diagrams

For a square planar Ni(II) complex, draw the energy level diagram splitting the d orbitals as follows (from low to high energy): \(d_{z^2}\), \(d_{xy}\), \(d_{yz}\), \(d_{xz}\), \(d_{x^2-y^2}\). For a tetrahedral complex, the orbitals split into two groups: \(d_{xy}\), \(d_{yz}\), \(d_{xz}\) at lower energy and \(d_{z^2}\), \(d_{x^2-y^2}\) at higher energy.

Determine Orbital Population

Ni(II) has a 3d\(^8\) configuration. In a square planar complex, the electrons fill lower energy levels first, often leading to the \(d_{x^2-y^2}\) orbital being unoccupied. In a tetrahedral complex, the electrons fill the three lower \(t_{2g}\) orbitals \(d_{xy}, d_{yz}, d_{xz}\), and distribute among the \(e_g\) \(d_{z^2}, d_{x^2-y^2}\) orbitals as needed.

Analyze Magnetic Properties

Paramagnetism indicates unpaired electrons, while diamagnetism indicates all paired electrons. The square planar complex usually accommodates the electrons such that no unpaired electrons remain, leading to a diamagnetic state. Conversely, some unpaired electrons in tetrahedral complexes cause paramagnetism.

Determine Geometries Based on Magnetic Properties

Since \(\mathrm{NiCl}_{2} \mathrm{~L}_{2}\) is paramagnetic, it is likely tetrahedral, having unpaired electrons. \(\mathrm{Ni}(\mathrm{NCS})_{2} \mathrm{~L}_{2}\) is diamagnetic, suggesting a square planar geometry.

Draw Structures and Address Dipole Moments

Draw a tetrahedral structure for \(\mathrm{NiCl}_{2} \mathrm{~L}_{2}\), and a square planar structure for \(\mathrm{Ni}(\mathrm{NCS})_{2} \mathrm{~L}_{2}\). The square planar complexes are generally not dipolar due to symmetric charge distribution, while tetrahedral complexes can have a net dipole moment.

Key concepts

Nickel(II) complexes

Nickel(II) complexes involve the participation of Nickel in a +2 oxidation state. These complexes can exhibit different combinations of ligands like \((\mathrm{Cl}^{-}\) or N-bonded \((\mathrm{NCS}^{-}\) along with a neutral ligand such as Triphenylphosphine (\(P(Ph)_{3}\)). This allows the science behind the formation of either a square planar or tetrahedral complex based on ligand field stabilization. Nickel, being a transition metal, typically forms coordination complexes due to its partially filled d orbitals, compelling Nickel(II) complexes to adopt specific geometries influenced by the type of ligands. Furthermore, certain complexes adjust their geometries in response to ligand field strength, leading to consequences in magnetism and dipole moment which we will explore in succeeding sections.

Square planar and tetrahedral geometries

The geometry of Nickel(II) complexes can often be square planar or tetrahedral. This depends largely on the nature of the ligands and electronic configuration. For Nickel(II), the common geometries are:- **Square Planar Geometry:** This configuration often arises when the complex is influenced by strong field ligands like \(\mathrm{NCS}^{-}\). It results in a distinctive crystal field splitting, lowering certain d orbitals more than others, often producing a diamagnetic complex due to all electron pairs being paired.- **Tetrahedral Geometry:** Commonly resulting with weaker field ligands such as \(\mathrm{Cl}^{-}\), which causes lesser splitting. It tends to accommodate unpaired electrons leading to a paramagnetic mix favoring unpaired electrons in \(t_{2g}\) orbitals.Each of these geometries leads to distinct chemical and physical properties, such as dipole moments and catalytic activity.

Paramagnetism and diamagnetism

Nickel(II) complexes showcase interesting magnetic properties due to varied electron pairings in different geometries. - **Paramagnetism:** This property arises when complexes have one or more unpaired electrons. For example, \(\mathrm{NiCl}_{2} \mathrm{L}_{2}\) typically shows paramagnetism due to the tendency of tetrahedral complexes to have unpaired spins unwound by weaker ligand fields.- **Diamagnetism:** Occurs when electrons are fully paired within the complex. For instance, \(\mathrm{Ni(NCS)}_{2} \mathrm{L}_{2}\) is often diamagnetic, representing square planar complexes with stronger ligand fields encouraging paired electronic states.Understanding these properties aids in not only predicting the behavior of these complexes in magnetic fields but also in determining their geometry and reactivity.

Orbital splitting diagrams

Orbital splitting diagrams are essential visuals in describing how the energy levels of metal d orbitals are split in different chemical environments.- **Square Planar Splitting:** For square planar arrangements, the d orbitals are impacted uniquely, with \(d_{x^{2}-y^{2}}\) orbital raised the highest in energy, often left unoccupied in a 3d\({}^{8}\) configuration due to \(\mathrm{Ni^{2+}}\) having an electron filling mechanism that favors lower energy orbitals like \(d_{z^{2}}, d_{xy}\), leading to distinct electron pairing pattern.- **Tetrahedral Splitting:** The d orbital splitting in tetrahedral complexes features a less extensive separation, resulting in \(t_{2g}\) orbitals \(d_{xy}, d_{yz}, d_{xz}\) being at a lower energy than \(e_{g}\) orbitals, influencing the paramagnetic property.These diagrams are critical to visualize and predict the geometry and magnetic properties of these complexes.