Out of `CO` and `CO_(2)`, `"………….."` forms complexes.

Answer

To determine which of the two compounds, carbon monoxide (CO) or carbon dioxide (CO₂), forms complexes, we can analyze their structures and bonding characteristics.

### Step-by-Step Solution:

1. **Analyze the Structure of CO**:
- Carbon monoxide (CO) consists of a carbon atom triple-bonded to an oxygen atom. The carbon atom has a lone pair of electrons that can be used for bonding.
- This lone pair allows CO to act as a Lewis base, meaning it can donate its lone pair to a metal atom, forming coordinate bonds.

2. **Identify Complex Formation in CO**:
- Due to the presence of a lone pair on carbon, CO can form complexes with various metal ions. For example, in hemoglobin, CO can bind to the iron (Fe) in the heme group, forming a complex.
- Another example is nickel carbonyl (Ni(CO)₄), where CO acts as a ligand and forms a complex with nickel.

3. **Analyze the Structure of CO₂**:
- Carbon dioxide (CO₂) has a linear structure with carbon double-bonded to two oxygen atoms. In this molecule, carbon does not have any lone pairs of electrons available for donation.
- As a result, CO₂ cannot act as a Lewis base and does not have the ability to form coordinate bonds with metal atoms.

4. **Conclusion**:
- Based on the analysis, carbon monoxide (CO) forms complexes due to its ability to donate a lone pair of electrons, while carbon dioxide (CO₂) does not form complexes because it lacks lone pairs for donation.

### Final Answer:
Out of CO and CO₂, **CO forms complexes**.