Question

Calculate the oxidation number of the central atom in $\left[ \text{Fe(CO}{{\text{)}}_{\text{5}}} \right]$ .

Answer 1

The charge on the complex is equal to the sum of the oxidation state of the metal centre and the charges on the ligands. The carbonyl group is a neutral ligand. The oxidation state of the carbonyl group is zero.

Complete answer:
We have the $\text{Fe(CO}{{\text{)}}_{\text{5}}}$ complex. The oxidation number of the $\text{Fe}$ in the complex can be calculated as follows:
Let consider the charge on the $\text{Fe}$ atom as the x.
The $\text{Fe(CO}{{\text{)}}_{\text{5}}}$ complex has the zero charge on the complex. Therefore,
$\text{Charge on complex= Charge on Fe + 5 (Charge on carbonyl ligand)}$
Or $\text{0 = Charge on Fe + 5 (0)}$
Since the carbonyl is a neutral ligand. Therefore charge on the carbonyl is zero.
On further simplification. We have,
$\text{Charge on Fe =0}$
Thus, the oxidation number or charges on the iron $\text{Fe}$ atom in $\text{Fe(CO}{{\text{)}}_{\text{5}}}$ the complex is zero.

Additional information:
-The geometry of $\text{Fe(CO}{{\text{)}}_{\text{5}}}$ the complex: The iron in the $\text{Fe(CO}{{\text{)}}_{\text{5}}}$ complex is surrounded by the five carbonyl ligands. $\text{Fe(CO}{{\text{)}}_{\text{5}}}$ The complex has trigonal bipyramidal geometry.
-Carbonyl is a neutral ligand.
In the preparation of metal carbonyls, metal is considered as the centre of a positive charge. It acts as the electron pair acceptor. The carbonyl ligands act as the ligand. They donate the electron to the metal. Primarily, carbonyl donates the electron pair to the metal which results in the formation of a metal-ligand bond. The metal seems to acquire a negative charge. The metal has the filled d-orbital which tends to donate the pair of electrons back to the carbonyl ligand. Ligand has empty p-orbital. It readily accepts the electron pair from the metal. Ligands accept this electron pair in pi-antibonding molecular orbitals to form a metal-ligand pi bond. This is called back bonding. Since the electron pair is accepted by metal and an equal number of the electron is donated by the metal thus the net effect is zero. Therefore, the carbonyl is neutral ligands.

Note:
The compounds which follow the eighteen electron rule (EAN) have the zero charge on them. Here the $\text{Fe(CO}{{\text{)}}_{\text{5}}}$ complex has a total of 36 electrons. $\text{Fe}$ attains the nearest noble gas configuration. Thus the $\text{Fe(CO}{{\text{)}}_{\text{5}}}$ complex has zero charges.