Some unidentate ligands have more than one donor atom and these may coordinate to the metal ion through either of the two atoms. These types of ligands are called ambidentate ligands. Some examples are:-

(i) M  ← NO₂                                                  Nitro group
   ( N is the donor atom )
     M    ←   ONO                                             Nitro group
   ( O is the donor atom )

(ii) M   ←  SCN                                              S - Thiocyanato
    ( S is the donor atom )
      M    ←     NCS
    ( N is the donor atom )                             N - Thiocyanato

(iii) M   ← CN                                                  Cyano
     ( C is the donor atom )
       M   ←  NC                                                  Isocyano
     ( N is the donor atom )
Complexes of these ligands yield linkage isomers.

Bridging Ligands:
Some monodentate ligands can simultaneously coordinate to two or more metal atoms. As a result, the ligand acts as a bridge between different metal atoms. It is, therefore, called a bridging ligand. The resulting Compound is known as a bridged complex. For example, iron (III) chloride exists as dimmer. Fe₂CI₆ in which two chlorine atoms act as bridging ligands.
Carboxlates, RCOO^- and sulphates, SO₄^2- are also among the few ligands which form bridged complexes:

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