why is NO2 bent while [NO2]+ is linear?

In NO₂, the central N atom has 5 valence electrons. 2 valence electrons form an N=O double bond, 2 valence electrons form an N→O dative bond, and 1 single electron is left (with resonance). According VSEPR theory, the shape of the 3 groups of valence electrons is triangular, and the bond angle of O=N→O is bent.

In [NO₂]⁺, one electrons is removed from NO₂, and thus and central N atom has 4 valence electrons. The 4 valence electrons form 2 N=O double bonds. According to VSEPR theory, the shape of the 2 groups of electrons are linear, and thus the bond angle of O=N=O is 180°.

The structures of NO₂ and [NO₂]⁺ are shown below :
https://c1.staticflickr.com/9/8534/28119093854_c57e119b83_o.png