Which molecule/ion among the following cannot act as a ligand in complex compounds ?

Options

A. `CH_(4)`
B. `CN^(-)`
C. `Br^(-)`
D. `CO`

Answer

To solve the question "Which molecule/ion among the following cannot act as a ligand in complex compounds?", we will analyze the given options and determine which one does not have the ability to donate an electron pair to a metal ion.

### Step-by-Step Solution:

1. **Understanding Ligands**:
- Ligands are molecules or ions that can donate a pair of electrons to a metal ion to form a coordinate bond. This means that for a substance to act as a ligand, it must have at least one lone pair of electrons available for donation.

2. **Analyzing the Options**:
- Let's consider the options provided (though they are not specified in the question, we will focus on CH4 based on the transcript).
- **Option A: CH4 (Methane)**:
- CH4 is a simple hydrocarbon with a tetrahedral structure. It consists of one carbon atom bonded to four hydrogen atoms.
- **Electron Pair Availability**: CH4 has no lone pairs of electrons; all electrons are involved in bonding with hydrogen atoms. Therefore, it cannot donate an electron pair.
- **Option B, C, D**:
- Although we do not have the specific identities of these options, we can infer from the transcript that they likely include molecules or ions such as CN⁻ (cyanide), Br⁻ (bromide), and CO (carbon monoxide), all of which have lone pairs of electrons and can act as ligands.

3. **Conclusion**:
- Since CH4 does not have any lone pairs of electrons to donate, it cannot act as a ligand in complex compounds. Thus, the correct answer to the question is CH4.

### Final Answer:
- **CH4 cannot act as a ligand in complex compounds.**