= Solution
The <nitrite> ion is an <ambidentate ligand>: its <Lewis structures> leave donor <lone pairs> on both <nitrogen> and <oxygen>. In the N-bound mode, $\mathrm{M{-}N(O)_2}$, the <nitrogen> <lone pair> makes a <sigma bond> to the metal, while a suitable unoccupied <nitrite> <molecular orbital> can accept metal $d$ <electron> density by <pi backbonding>. Thus \b[N-bound nitro is a sigma donor and pi acceptor]. The <nitrite> group remains bent at <nitrogen>; N coordination does not make it a linear nitrosyl.
In the O-bound mode, $\mathrm{M{-}O{-}N{=}O}$, an <oxygen> <lone pair> makes the metal–<oxygen> <sigma bond>. A second occupied <oxygen> orbital can donate sideways into an available metal orbital: \b[O-bound nitrito is a sigma and pi donor], rather than the strong pi acceptor of the N-bound description. <oxygen>'s greater <electronegativity> lowers its donor orbital energies, but the occupied <oxygen> orbitals still explain its pi-donor classification. Actual mixing depends on metal <oxidation state> and geometry.
A further possibility is <bidentate coordination>, $\mathrm{M}(\kappa^2\!\text{-}O,O\text{-}\mathrm{NO_2})$, with both <oxygen> atoms attached to one metal. Each attachment has oxygen-to-metal sigma donation; additional pi donation depends on orbital alignment. <nitrite> can also be a <bridging ligand> between metals. Neither alternative should be confused with <linkage isomerism>, which specifically compares attachment through different atoms at the same binding site.
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