In some books the examples for positive ligands are given as: $\ce{NO+}$, $\ce{NO2+}$, $\ce{NH2-NH3+}$, $\ce{NH4+}$, $\ce{H3O+}$, $\ce{PH4+}$. I can understand about the first three examples, but wondering about the last three examples.
It would be great if someone can explain if the latter three are positive ligands and if yes, please explain how it coordinates with metals.
