  We report on a Ni L$_{2,3}$ edges x-ray absorption spectroscopy (XAS) study
in $R$NiO$_3$ perovskites. These compounds exhibit a metal to insulator ($MI$)
transition as temperature decreases. The L$_{3}$ edge presents a clear
splitting in the insulating state, associated to a less hybridized ground
state. Using charge transfer multiplet calculations, we establish the
importance of the crystal field and 3d spin-orbit coupling to create a
mixed-spin ground state. We explain the $MI$ transition in $R$NiO$_3$
perovskites in terms of modifications in the Ni$^{3+}$ crystal field splitting
that induces a spin transition from an essentially low-spin (LS) to a
mixed-spin state.
