Which of the following ligands is most likely to form high-spin complexes in octahedral fields?

\(Cl^-\)

The correct answer is option 1. \(Cl^-\).

Among the ligands listed, \(Cl^-\) (chloride ion) is most likely to form high-spin complexes in octahedral fields. In coordination chemistry, the term "high-spin" refers to a configuration where electrons occupy higher energy orbitals before pairing in lower energy orbitals. This typically occurs when the crystal field splitting energy (\( \Delta_o \)) is relatively small, allowing the electrons to occupy higher energy orbitals and leading to a high-spin configuration.

Factors Influencing High-Spin Complex Formation:

Crystal Field Splitting Energy (\( \Delta_o \)):

For octahedral complexes, the magnitude of \( \Delta_o \) determines whether a complex will be high-spin or low-spin. If \( \Delta_o \) is small (weak field ligands or weak crystal field), electrons preferentially occupy the higher energy orbitals, resulting in a high-spin configuration.

Nature of Ligands:

Ligands can be classified as strong field ligands or weak field ligands based on their ability to cause crystal field splitting. Weak field ligands (like \( \text{Cl}^- \)) produce a small \( \Delta_o \), favoring high-spin complexes. Strong field ligands produce a large \( \Delta_o \), favoring low-spin complexes.

Analysis of Ligands:

1. \(Cl^-\) (Chloride ion):

Chloride ion is a weak field ligand. It generates a small crystal field splitting energy (\( \Delta_o \)), allowing for high-spin configurations in octahedral complexes.

2. \(OH^-\( (Hydroxide ion):

Hydroxide ion is a moderately weak field ligand. It produces a slightly higher \( \Delta_o \) compared to chloride, so complexes with \(OH^-\) can exhibit both high-spin and low-spin configurations depending on the metal and other factors.

3. \(C_2O_4^{2-}\) (Oxalate ion):

Oxalate ion is a moderately strong field ligand. It induces a larger \( \Delta_o \), favoring low-spin configurations in octahedral complexes.

4. \(CN^-\) (Cyanide ion):

Cyanide ion is a strong field ligand. It creates a large \( \Delta_o \), which strongly favors low-spin configurations in octahedral complexes.

Conclusion:

Based on the criteria that high-spin complexes form when \( \Delta_o \) is small, chloride ion \((Cl^-)\) is most likely to form high-spin complexes among the options listed. Therefore, the correct answer is \(Cl^-\).