Question

Identify number of moles of donor atoms in 2n mole of SCN.

• A. 3n
• B. 6n
• C. 4n
• D. n

Hint:

Logic Tip: Do not confuse "donor atoms" with "denticity". While SCN is typically a monodentate ligand (denticity = 1, binds through one atom at a time per metal center), it possesses 2 actual donor atoms on the molecule. The question asks for the literal count of donor atoms present in the formula.

Answer 1

The Correct Option is C

Concept:
A ligand is an ion or molecule that donates a pair of electrons to a central metal atom or ion to form a coordinate covalent bond. An ambidentate ligand is a special type of monodentate ligand that has more than one potential donor atom, though it usually binds through only one at a time. The question asks for the total \textit{available} donor atoms present within the specified amount of the ligand.
Step 1: Analyze the structure of the SCN ligand.
The thiocyanate ion ($SCN^-$) is an ambidentate ligand. It contains two distinct atoms equipped with lone pairs of electrons capable of being donated to a metal center:

• The Sulfur (S) atom.
• The Nitrogen (N) atom.

Therefore, each single $SCN^-$ ion possesses exactly $2$ donor atoms.
Step 2: Calculate the moles of donor atoms.
Since 1 mole of $SCN^-$ contains 2 moles of donor atoms (1 mole of S and 1 mole of N), we simply scale this by the given quantity. Amount of ligand given = $2n$ moles. Total moles of donor atoms = (Amount of ligand) $\times$ (Donor atoms per ligand) Total moles of donor atoms = $2n \times 2 = 4n$ moles.