  Rare-earth nickelates form an intriguing series of correlated perovskite
oxides. Apart from LaNiO3, they exhibit on cooling a sharp metal-insulator
electronic phase transition, a concurrent structural phase transition and a
magnetic phase transition toward an unusual antiferromagnetic spin order.
Appealing for various applications, full exploitation of these compounds is
still hampered by the lack of global understanding of the interplay between
their electronic, structural and magnetic properties. Here, we show from
first-principles calculations that the metal-insulator transition of nickelates
arises from the softening of an oxygen breathing distortion, structurally
triggered by oxygen-octahedra rotation motions. The origin of such a rare
triggered mechanism is traced back in their electronic and magnetic properties,
providing a united picture. We further develop a Landau model accounting for
the evolution of the metal-insulator transition in terms of the $R cations and
rationalising how to tune this transition by acting on oxygen rotation motions.
