arenium ion
/ uh-REE-nee-um /
Picture the moment in the middle of an electrophilic aromatic substitution, just after the electrophile has grabbed onto the ring but before anything is lost. One ring carbon now has both a hydrogen and the new group attached, so it has gone from flat sp2 to a bulkier sp3 corner. The remaining five carbons share a positive charge among them. This fleeting, positively charged, partly-broken-up ring is the arenium ion — the key intermediate every EAS reaction passes through.
What keeps this intermediate from being hopelessly unstable is resonance. The positive charge is not stuck on one carbon; it is spread over three of the five remaining ring carbons (the two ortho positions and the para position relative to where the electrophile attached). Drawing the three resonance structures shows the charge sliding around the ring, and the real species is the single hybrid of all three — that delocalization is exactly what makes the arenium ion stable enough to form, even though it is no longer aromatic. It is sometimes called the sigma-complex (because a new sigma bond has formed) or the Wheland intermediate.
The arenium ion is the hinge of the whole mechanism. Forming it is the slow, rate-determining step, because it costs the ring its aromaticity. But it is only a valley on the energy landscape, not the destination: the ring is desperate to get its aromaticity back, so a base quickly removes the proton from that sp3 carbon, the electrons flow back in, and the aromatic ring returns. Understanding which positions carry the positive charge in this intermediate is also what lets chemists predict ortho/para versus meta directing — substituent effects are really just statements about which arenium ion is more stable.
When NO2(+) attaches to benzene, the resulting arenium ion can be drawn with the (+) charge on the carbon ortho to NO2, then meta, then para — three resonance contributors. The actual ion is the hybrid, with positive character at the two ortho and the para carbons.
Three resonance pictures, one real hybrid — the charge sits on the ortho and para carbons.
The three resonance structures are not three different molecules the ion flickers between — they are three ways of drawing one delocalized species. Whenever a substituent's lone pair or pi system can add a fourth resonance structure that further spreads the charge, the arenium ion becomes more stable, which is the root of all directing effects.