Question

Which molecule/ion among the following cannot act as a ligand in complex compounds?
A. $CO$
B. $C{{H}_{4}}$
C. $C{{N}^{-}}$
D. $B{{r}^{-}}$

Answer 1

Ligand is a molecule or ion that forms a bond with the central metal atom and forms a complex. The ligand should contain a lone pair of electrons or unpaired electrons in their orbitals to form a bond with the central metal atom to form coordination complexes.

Complete step by step answer:
- In the question it is given that which molecule/ion among the given options cannot act as a ligand in complex compounds.
- We have to find the molecule which does not act as a ligand in the given options.
- Coming to the given options, option A CO. We know that the oxygen atom has two lone pairs of electrons. Then carbon monoxide acts as a good ligand and forms a bond with the central metal atom.
- Coming to option B, methane. Methane does not contain any lone pair of electrons and unpaired electrons in its valence shell. So, methane does not act as a ligand.
- Coming to the option C, $C{{N}^{-}}$ . Cyanide ion has extra electrons in its valence shell then it will donate the electrons to the metal atom very easily. So, cyanide acts as a good ligand.
- Coming to the option D, $B{{r}^{-}}$ . Bromide ion has extra electrons in its valence shell then it will donate the electrons to the metal atom very easily. So, bromide acts as a good ligand.
- Therefore methane does not act as a ligand due to the absence of lone pair and valence electrons and it does not act as a ligand.

So, the correct option is B.

Note: In coordination complexes the metal is going to accept the electrons donated by the ligand and forms a complex. If the ligands donate electrons very easily then the ligand is called strong ligand and if the ligand donates electrons with some effort then the ligands are called weak ligands.