Question

For the complex \(\mathrm{[Fe(en)_2Cl_2]Cl}\), identify :

(a) the oxidation number of iron.

(b) the hybridization and the shape of the complex.

(c) the magnetic behaviour of the complex.

(d) whether there is an optical isomer of the complex ? If so draw its structure.

(e) IUPAC name of the complex.

(At. no. of Fe = 26)

Hint:

Charge on the cation is +1, so Fe = +3; en is a strong field ligand giving \(d^2sp^3\).
Only the cis form lacks a plane of symmetry.

Answer 1

Step 1: Understanding the Question:
The formula is \(\mathrm{[Fe(en)_2Cl_2]Cl}\). The part in square brackets is the complex ion. The last \(\mathrm{Cl^-}\) is the counter ion outside the bracket, so the complex ion has a charge of +1.
Here en is ethane-1,2-diamine, a neutral ligand that binds through two nitrogen atoms (bidentate).

Step 2: (a) Oxidation number of iron:
Let the oxidation number of Fe be \(x\). Charges: en = 0 (two of them), \(\mathrm{Cl^-}\) = -1 (two inside the bracket).
\[ x + 2(0) + 2(-1) = +1 \Rightarrow x = +3 \]
So iron is in the +3 state.

Step 3: (b) Hybridisation and shape:
Fe has atomic number 26, so \(\mathrm{Fe^{3+}}\) is \(3d^5\). The coordination number is 6 (two en x 2 donor atoms + two Cl).
Ethane-1,2-diamine is a strong field ligand, so it forces the five d electrons to pair up in the lower three d orbitals (\(t_{2g}^5\)). This frees two 3d orbitals.
Two 3d, one 4s and three 4p orbitals mix to give six \(d^2sp^3\) hybrid orbitals. The complex is inner orbital, and the shape is octahedral.

Step 4: (c) Magnetic behaviour:
With the pairing in \(t_{2g}^5\) there are two paired sets and one unpaired electron.
Magnetic moment \(\mu = \sqrt{n(n+2)} = \sqrt{3} \approx 1.73\) BM, so the complex is paramagnetic.

Step 5: (d) Optical isomerism:
Yes. The complex is of the type \(\mathrm{[M(AA)_2X_2]}\). Its trans form has a plane of symmetry and is optically inactive.
The cis form has no plane of symmetry. Its mirror image cannot be superimposed on it, so it exists as a pair of enantiomers (d and l forms).
Structure of the cis form in words: iron is at the centre of an octahedron. The two Cl atoms occupy adjacent (cis) corners. Each en ring spans two adjacent corners with its two N atoms, and the two rings are arranged like a propeller. The mirror image of this arrangement (rings twisted the other way) is the second optical isomer.

Step 6: (e) IUPAC name:
Ligands are named in alphabetical order: chlorido (c) comes before ethane-1,2-diamine (e), written bis because it is a ring ligand. The metal is in a cation, so it keeps the name iron, followed by the oxidation state. The outside ion is named last.
Name: dichloridobis(ethane-1,2-diamine)iron(III) chloride.

Final Answer:
(a) +3. (b) \(d^2sp^3\), octahedral. (c) Paramagnetic (one unpaired electron). (d) Yes, the cis form is optically active and exists as d and l isomers. (e) Dichloridobis(ethane-1,2-diamine)iron(III) chloride.