Question

Describe an experiment that would determine whether nickel in \(\mathrm{K}_{2}\left[\mathrm{NiCl}_{4}\right]\) is square-planar or tetrahedral.

Short answer

Use UV-Visible spectroscopy, magnetic susceptibility, or X-ray crystallography to determine geometry.

Step-by-step solution

Understand the Geometry Types

Nickel can be arranged in two primary geometries: square-planar and tetrahedral. A square-planar geometry means the atoms form a square with the metal center in the plane. Tetrahedral geometry means the atoms form a tetrahedron around the metal center.

Consider Spectroscopic Techniques

Use UV-Visible Spectroscopy to identify the geometry. Square-planar complexes usually show distinct absorbance bands compared to tetrahedral complexes due to differences in d-orbital splitting. Square-planar complexes exhibit larger d-d transition energy, resulting in bands at a shorter wavelength.

Evaluate Magnetic Properties

Measure the magnetic susceptibility of the compound. Square-planar complexes with nickel often have no unpaired electrons, making them diamagnetic, while tetrahedral complexes display paramagnetism due to unpaired electrons.

Perform X-ray Crystallography

Conduct X-ray crystallography, which directly reveals the molecular geometry. The arrangement of atoms (tetrahedral vs. square-planar) will be clearly visible from the crystal structure analysis, confirming the geometry conclusively.

Key concepts

Molecular Geometry

Understanding molecular geometry is crucial in coordination chemistry as it describes the three-dimensional arrangement of atoms around a central metal ion. This determines many of the compound's physical and chemical properties. For the compound \(\mathrm{K}_{2}[\mathrm{NiCl}_{4}]\), the nickel ion can adopt either a square-planar or a tetrahedral geometry.

In a square-planar geometry, atoms lie at the corners of a square surrounding the central metal atom found within the plane. This geometry is common in transition metals, especially with a coordination number of four, like nickel in low-spin states. Tetrahedral geometry, in contrast, resembles a pyramid with a triangular base; atoms reside at the four corners of a tetrahedron. This arrangement is more typical of high-spin complexes and results in different chemical behavior. Knowing the precise geometry helps predict reactivity, stability, and the nature of bonding in metal complexes.

UV-visible Spectroscopy

UV-visible spectroscopy is a powerful tool used to analyze the electronic transitions in complexes, which provides insight into their molecular geometry. When light interacts with a compound, electrons can absorb energy and move to higher energy levels. This process produces an absorption spectrum.

For \(\mathrm{K}_{2}[\mathrm{NiCl}_{4}]\), the square-planar and tetrahedral geometries would result in different patterns. Specifically, square-planar structures show larger d-d transitions. This is due to greater splitting of the d-orbitals compared to tetrahedral geometries, leading to absorbance at shorter wavelengths. Such spectroscopic differences can thus help distinguish between the two geometries by analyzing the pattern and position of the absorption bands.

Magnetic Susceptibility

Magnetic susceptibility measurements provide insights regarding the electronic configurations of complexes, which can indicate molecular geometry. By investigating whether electrons are paired or unpaired, one can assess the magnetic properties of a sample.

In the case of \(\mathrm{K}_{2}[\mathrm{NiCl}_{4}]\), a square-planar geometry often suggests a diamagnetic nature, where all electrons are paired, resulting in no net magnetic field. In contrast, a tetrahedral complex might exhibit paramagnetism due to the presence of unpaired electrons, which generate a measurable magnetic moment.

• Diamagnetic: All electrons paired; no net magnetic field.
• Paramagnetic: Some unpaired electrons; displays magnetic properties.

Measuring the compound’s magnetic susceptibility will thus help infer its geometry based on these magnetic properties.

X-ray Crystallography

X-ray crystallography is the definitive method for determining the molecular geometry of a complex. This technique involves directing X-ray beams onto a crystalline sample to produce diffraction patterns that reveal the spatial arrangement of atoms.

For \(\mathrm{K}_{2}[\mathrm{NiCl}_{4}]\), using X-ray crystallography can unambiguously establish whether the compound is square-planar or tetrahedral. The method accurately maps the positions of atoms in the crystal lattice, providing clear and discerning visuals of the molecular structure. Such precision makes it an invaluable tool in coordination chemistry for validating molecular geometries beyond theoretical predictions and spectroscopic or magnetic evidence.