  The ground-state orbital occupancy of the Ru$^{4+}$ ion in
Ca$_{2-x}$La$_x$RuO$_4$ [x=0, 0.05(1), 0.07(1) and 0.12(1)] was investigated by
performing X-ray absorption spectroscopy (XAS) in the vicinity of the O K edge
as a function of angle between the incident beam and the surface of the
crystals. A minimal model of the hybridization between the O 2p states probed
at the K edge and the Ru 4d orbitals was used to analyze the XAS data, allowing
the ratio of hole occupancies $n_{xy}/n_{yz,zx}$ to be determined as a function
of doping and temperature. For the samples displaying a low-temperature
insulating ground-state ($x\leq0.07$), $n_{xy}/n_{yz,zx}$ is found to increase
significantly with increasing doping. For x=0.12, which has a metallic
ground-state, the XAS spectra are found to be independent of temperature, and
not to be describable by the minimal hybridization model. To understand the
origin of the evolution of the electronic structure across the phase diagram,
we have performed theoretical calculations based on a model Hamiltonian,
comprising electron-electron correlations, crystal field ($\Delta$) and
spin-orbit coupling ($\lambda$), of a Ru-O-Ru cluster. Our calculations of the
Ru hole occupancy as a function of $\Delta/\lambda$ establish that the
enhancement of $n_{xy}/n_{yz,zx}$ is driven by significant changes to the
crystal field as the tetragonal distortion of the RuO$_6$ octahedral changes
from compressive to tensile with La doping. It also shows that the hole
occupancy of the O 2p and Ru 4d orbitals display the same trend as a function
of $\Delta/\lambda$, thus validating the minimal hybridization model. In
essence, our results suggest that the predominant mechanism driving the
emergence of the low-temperature metallic phase in La doped Ca$_2$RuO$_4$ is
the structurally induced redistribution of holes within the t2g orbitals,
rather that the injection of free carriers.
