Pure Crystal field theory consider ligands as which of the following?

Negative point charges

The correct answer is option 2. Negative point charges.

In Crystal Field Theory (CFT), the interaction between the metal ion and the ligands is primarily viewed through an electrostatic lens. Here's a detailed explanation:

Electrostatic Interaction:  In CFT, ligands are treated as negative point charges. This is because ligands typically donate electron pairs to the central metal ion, forming coordinate covalent bonds. These electron pairs come from the lone pairs on the ligand atoms. As a result, ligands acquire a partial negative charge, which interacts electrostatically with the positively charged metal ion.

Effect on Metal Ion's d-Orbitals: The electrostatic interaction between the metal ion and the ligands influences the energy levels of the metal ion's d-orbitals. The d-orbitals experience a distortion in energy due to the presence of the ligands' negative charges. This distortion leads to a splitting of the d-orbitals into different energy levels. In an octahedral coordination geometry, for example, the d-orbitals split into two sets: a lower energy set of three orbitals (denoted as \(t_{2g}\)) and a higher energy set of two orbitals (denoted as \(e_g\)). This splitting is known as the Crystal Field Splitting.

Spectroscopic Consequences: The Crystal Field Splitting affects the absorption of light by transition metal complexes. When light passes through a solution of a transition metal complex, it can be absorbed if its energy matches the energy difference between the split d-orbitals. This absorption of light results in characteristic electronic spectra, often observed as color in transition metal complexes. The color observed in transition metal complexes arises from electronic transitions between the split d-orbitals, known as d-d transitions. These transitions involve the promotion of electrons from lower energy \(t_{2g}\) orbitals to higher energy \(e_g\) orbitals.

Limitations:

While CFT provides valuable insights into the electronic structure and spectroscopic properties of transition metal complexes, it does have limitations. For instance, CFT does not adequately explain the color of complexes with unusual geometries or the magnetic properties of transition metal complexes. To address these limitations, additional theories such as Ligand Field Theory (LFT) and Molecular Orbital Theory (MO) are often employed.

In summary, in Crystal Field Theory (CFT), ligands are considered as negative point charges due to their partial negative charge resulting from the donation of electron pairs to the central metal ion. This electrostatic interaction influences the energy levels of the metal ion's d-orbitals, leading to the characteristic splitting and spectroscopic properties observed in transition metal complexes.