Why nitro group is called an ambident group?

It can bond through either the N or O atoms  

Explanation:

An ambident ("both teeth") group is a group that can attach to another by either of two atoms.

A nitro group can bond through either N or O.

Organic chemistry

In organic chemistry, this property gives rise to functional group isomers.

For example, we can have either nitromethane (CH₃NO₂, Fig. 1) or methyl nitrite (CH₃ONO, Fig. 2)

Inorganic chemistry

The ambidentate property of the nitro group gives rise to linkage isomers in inorganic coordination complexes.

For example, we can have either a pentachloronitroiron(III) cation (Fig. 3) or a pentachloronitritoiron(III) cation (Fig. 4)

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