  $RE$NiO$_3$ is a negative charge transfer energy system and exhibits a
temperature-driven metal-insulator transition (MIT), which is also accompanied
by a bond disproportionation (BD) transition. In order to explore how hole
doping affects the BD transition, we have investigated the electronic structure
of single-crystalline thin films of Nd$_{1-x}$Ca$_x$NiO$_3$ by synchrotron
based experiments and {\it ab-initio} calculations. For a small value of $x$,
we find that the doped holes are localized on one or more Ni sites around the
dopant Ca$^{2+}$ ions, while the BD state for the rest of the lattice remains
intact. The effective charge transfer energy ($\Delta$) increases with Ca
concentration and the formation of BD phase is not favored above a critical
$x$, suppressing the insulating phase. Our present study firmly demonstrates
that the appearance of BD mode is essential for the MIT and settles a
long-standing debate about the role of structural distortions for the MIT of
the $RE$NiO$_3$ series.
