Explanation:
Fe2(CO)9 is an organometallic compound consisting of two iron atoms and nine carbon monoxide (CO) ligands. The CO ligands can bind to the iron atoms in different ways, forming either triple bonds or double bonds.

Number of triply bonded C ≡ O units:

- Triply bonded C ≡ O units are formed when a carbon atom in a CO ligand forms a triple bond with an iron atom.
- Each CO ligand can form either one triple bond or one double bond.
- In Fe2(CO)9, there are nine CO ligands, so the maximum number of triply bonded C ≡ O units is nine.
- However, not all CO ligands will form triple bonds with iron atoms. Some may form double bonds instead.
- To find the actual number of triply bonded C ≡ O units, we need to subtract the number of doubly bonded C = O units from nine.
- Therefore, the number of triply bonded C ≡ O units is 9 - (number of doubly bonded C = O units).

Number of doubly bonded C = O units:

- Doubly bonded C = O units are formed when a carbon atom in a CO ligand forms a double bond with an iron atom.
- Each CO ligand can form either one triple bond or one double bond.
- In Fe2(CO)9, there are nine CO ligands, so the maximum number of doubly bonded C = O units is nine.
- However, not all CO ligands will form double bonds with iron atoms. Some may form triple bonds instead.

Conclusion:

- The difference between the number of triply bonded C ≡ O units and doubly bonded C = O units in Fe2(CO)9 is three.
- This means that there are three more triply bonded C ≡ O units than doubly bonded C = O units in the molecule.

In the molecular structure of Fe₂(CO)₉,, The difference between the number of triply bonded C ≡ O units and doubly bonded C = O units are shown below by structure