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

${\mathrm{CH}}_4$ cannot act as a ligand as it contains no lone pair(s) of electrons. Methane ${\mathrm{CH}}_4$​ cannot act as a ligand in complex compounds as all electrons in it are paired. There is no lone pair of electrons or pi electrons which can be donated to metal to form a bond.