In the R

{NiO}_{3}

series (R=La,Pr,Nd,Sm), the metal-insulator (M-I) transition temperature rises systematically as the size of the rare earth decreases and as the subsequent distortion from the ideal cubic perovskite increases. For R=La the system keeps its metallic character down to 1.5 K, while for R=Pr, Nd, and Sm electronic localization occurs at 135, 200, and 400 K, respectively. High-resolution neutron-powder-diffraction experiments have been performed to investigate the structural anomalies across the first-order M-I transition in the orthorhombic

{PrNiO}_{3}

and

{NdNiO}_{3}

compounds. The cell volume undergoes a subtle increase when the compounds become insulating, due to a slight increase of the Ni-O distances. This effect is accompanied by coupled tilts of

{NiO}_{6}

octahedra, which imply changes in the Ni-O-Ni angles (Δ

Θ_{O - Ni - O}

≊-0.5°) governing the transfer integral between Ni

e_{g}

and O 2p orbitals. These changes are sterically driven by the observed increase of the nickel-oxygen distances (Δ

d_{Ni - O}

≊+0.004 Å) in the insulating (low-temperature) phase. The results of valence-bond calculations suggest the existence of

{Ni}^{3 +}

(

d^{7}

) and

R^{3 +}

states for nickel and rare earth.

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