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

1.  NO

2.  NH${}_{4}^{+}$

3.  NH${}_{2}$CH${}_{2}$CH${}_{2}$NH${}_{2}$

4.  CO

A chelating ligand has two or more binding donor atoms to a single metal ion

eg.

Here (4) denotes binding site.

thiosulphato (${S}_{2}{O}_{3}^{2}$) is not a chelating ligand because geometrically it is

not favourable for (${S}_{2}{O}_{3}^{2}$) to chelate a metal ion.