Complex Fe(CO)x and the value of x

Introduction:
The complex Fe(CO)x is a coordination compound of iron and carbon monoxide, where x represents the number of CO ligands bound to the central iron atom. The value of x depends on the electronic configuration and the oxidation state of the central iron atom.

Explanation:
The electronic configuration of iron is [Ar]3d64s2, and it can exhibit different oxidation states, such as +2, +3, and +4. In the Fe(CO)x complex, the iron atom has a formal oxidation state of zero, which means that the total number of electrons contributed by the CO ligands must balance the number of valence electrons of iron.

For example, if the oxidation state of iron is +2, then the Fe(CO)x complex must have x = 4 CO ligands, because the iron atom has 8 valence electrons, and each CO ligand donates two electrons. Similarly, if the oxidation state of iron is +3, then the Fe(CO)x complex must have x = 5 CO ligands, because the iron atom has 7 valence electrons, and each CO ligand donates two electrons.

However, if the oxidation state of iron is +4, then the Fe(CO)x complex must have x = 6 CO ligands, because the iron atom has 6 valence electrons, and each CO ligand donates two electrons.

Conclusion:
Therefore, the value of x in the complex Fe(CO)x depends on the oxidation state of the central iron atom, and it can be determined by balancing the total number of electrons contributed by the CO ligands with the number of valence electrons of iron. In this case, the correct answer is option 'C', which corresponds to x = 5 CO ligands.