Which of the following species is unable to act as a ligand in the formation of a transition metal complex ion?
CH3NH3+\text{CH}_3\text{NH}_3^+CH3NH3+
CH3NH2\text{CH}_3\text{NH}_2CH3NH2
NH2CH2CH2NH2\text{NH}_2\text{CH}_2\text{CH}_2\text{NH}_2NH2CH2CH2NH2
C2O42−\text{C}_2\text{O}_4^{2-}C2O42−