Among the following species the one which causes the highest CFSE, $$\Delta_0$$ as a ligand is:

The question asks which ligand causes the highest crystal field stabilization energy (CFSE), denoted as Δ₀, among CN⁻, NH₃, F⁻, and CO. CFSE depends on the magnitude of the crystal field splitting, Δ₀, which varies with the ligand. Ligands that cause larger Δ₀ lead to higher CFSE.

To determine the ligand that causes the largest Δ₀, we refer to the spectrochemical series. This series orders ligands based on their ability to split the d-orbitals in an octahedral field, from weakest to strongest field. The standard spectrochemical series is:

I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO

Now, comparing the ligands given in the options:

• Option C: F⁻ is a weak field ligand and appears early in the series, causing small Δ₀.
• Option B: NH₃ is stronger than F⁻ and appears after F⁻ in the series, so it causes a larger Δ₀ than F⁻.
• Option A: CN⁻ is a strong field ligand and appears near the end of the series, causing large Δ₀.
• Option D: CO is the strongest field ligand in this list and appears after CN⁻ in the series, meaning it causes the largest Δ₀.

CO causes the highest splitting because it is a strong π-acceptor ligand due to its empty π* orbitals, which allow for effective back-bonding with the metal, leading to greater stabilization and larger Δ₀ compared to CN⁻, NH₃, and F⁻.

Therefore, CO causes the highest CFSE, Δ₀.

Hence, the correct answer is Option D.