CN^– is a strong field ligand as it is a pseudohalide ion. These ions are strong coordinating ligands and hence have the tendency to form σ-bond (from the pseudo halide to the metal) and ?-bond, (from the metal to pseudo halide)

Explanation:

Introduction:
In coordination chemistry, ligands are molecules or ions that bind to a central metal atom or ion to form a coordination complex. Ligands can be classified into strong field ligands and weak field ligands based on their ability to split the d-orbitals of the central metal ion.

Definition of a strong field ligand:
A strong field ligand is a ligand that causes a large splitting of the d-orbitals of the central metal ion. This results in the formation of low spin complexes, where the electrons are preferentially placed in the lower energy orbitals.

Reasons why CN– is a strong field ligand:
There are several reasons why CN– is considered a strong field ligand. The correct answer, option B, states that CN– is a pseudohalide. Let's explore this further:

1. Pseudohalide nature:
CN– is a pseudohalide because it has similar properties to halide ions (such as F–, Cl–, Br–, I–), even though it is not a halide itself. Pseudohalides are known to be strong field ligands.

2. Size and charge:
CN– is a small ligand with a negative charge. The small size allows for greater overlap between the metal d-orbitals and the ligand's π* orbital, resulting in a stronger bonding interaction. The negative charge also enhances the electron-donating ability of CN–.

3. Backbonding:
CN– has a lone pair of electrons on the carbon atom, which can participate in backbonding with the metal d-orbitals. This backbonding further strengthens the metal-ligand bond and increases the ligand's field strength.

4. Polarity:
CN– is a polar ligand due to the electronegativity difference between carbon and nitrogen. This polarity enhances the interaction between the ligand and the central metal ion, leading to a stronger field.

5. High spin complexes:
Contrary to option D, CN– actually forms low spin complexes with metal species. This is because it is a strong field ligand that causes a large splitting of the d-orbitals, favoring the placement of electrons in the lower energy orbitals.

Conclusion:
In conclusion, CN– is considered a strong field ligand due to its pseudohalide nature, small size, negative charge, ability to participate in backbonding, and polarity. These factors result in a large splitting of the d-orbitals and the formation of low spin complexes with metal species.