  We study a model for the metal-insulator (MI) transition in the rare-earth
nickelates RNiO$_3$, based upon a negative charge transfer energy and coupling
to a rock-salt like lattice distortion of the NiO$_6$ octahedra. Using exact
diagonalization and the Hartree-Fock approximation we demonstrate that
electrons couple strongly to these distortions. For small distortions the
system is metallic, with ground state of predominantly $d^8\ligand$ character,
where $\ligand$ denotes a ligand hole. For sufficiently large distortions
($\delta d_{\rm Ni-O} \sim 0.05 - 0.10\AA$), however, a gap opens at the Fermi
energy as the system enters a periodically distorted state alternating along
the three crystallographic axes, with $(d^8\ligand^2)_{S=0}(d^8)_{S=1}$
character, where $S$ is the total spin. Thus the MI transition may be viewed as
being driven by an internal volume "collapse" where the NiO$_6$ octahedra with
two ligand holes shrink around their central Ni, while the remaining octahedra
expand accordingly, resulting in the ($1/2,1/2,1/2$) superstructure observed in
x-ray diffraction in the insulating phase. This insulating state is an example
of a new type of charge ordering achieved without any actual movement of the
charge.
