Question

Which molecule/ion among the following can't be act as a ligand in complex compounds?

a) CO

b) CN^-

c) NH_3

d) CH_4

Solution

Carbon Monoxide (CO) forms complex with Nickel.
CN^- is also a strong ligand. Br^- also forms complex with metal ions.

Methane (CH_4) does not act as a ligand as it does not have any lone pair of electrons nor a negative charge.

Therefore, option (d) is correct.