Fe2(CO)9 Structure:
In Fe2(CO)9, there are two iron atoms (Fe) and nine carbon monoxide molecules (CO) present. The structure of Fe2(CO)9 can be visualized as follows:

Fe - CO - CO - CO - CO - CO - CO - CO - CO - CO - Fe

Explanation:
- The Fe2(CO)9 compound consists of two iron atoms (Fe) and nine carbon monoxide (CO) molecules.
- Each carbon monoxide molecule consists of one carbon (C) atom and one oxygen (O) atom bonded together.
- In the Fe2(CO)9 compound, the iron atoms are connected to each other through the carbon monoxide ligands.
- The carbon monoxide ligands act as bridges between the iron atoms, forming a linear structure.
- The Fe2(CO)9 compound can be represented as [Fe(CO)3]2, where each iron atom is bonded to three carbon monoxide ligands.
- In this representation, there are three carbon monoxide ligands between the iron atoms.
- The remaining six carbon monoxide ligands are bonded to each iron atom individually.
- Therefore, the correct answer is '3' as there are three carbon monoxide ligands present between the iron atoms in Fe2(CO)9.

Key Points:
- Fe2(CO)9 consists of two iron atoms and nine carbon monoxide molecules.
- Carbon monoxide ligands act as bridges between the iron atoms.
- Fe2(CO)9 can be represented as [Fe(CO)3]2.
- There are three carbon monoxide ligands between the iron atoms in Fe2(CO)9.