Which of the following species is not expected to be a ligand?

• A

  ${\mathrm{PH}}_3$

• B

  ${\mathrm{NO}}^{+}$

• C

  ${\mathrm{N}}_2{\mathrm{H}}_5^{+}$

• D

  $\stackrel{+}{\mathrm{N}}{\mathrm{H}}_4$