The metal-to-insulator transition (MIT) in the rare-earth nickelate series, RENiO₃, remains a central open question due to the complex interplay of electron–electron interactions that stabilize a negative charge-transfer, bond-disproportionate ground state^[1]. Interest in this system has been renewed by the recent discovery of superconductivity in nickelate thin films^[2]. We present Resonant Inelastic X-ray Scattering (RIXS) studies on nine compounds (RE = La, Pr, Nd, Sm, Gd, Dy, Y, Ho, Er)^[3]. Our results reveal that the MIT is strongly influenced by a complex crystal-field environment. To interpret the data, we compute Wannier orbitals to constrain model parameters and perform two-site exact diagonalization calculations^[4], enabling quantitative modeling of both XAS and RIXS spectra. From these, we extract energy scales for charge transfer (△), hopping integrals and bond disproportionation^[5]. We discuss the implications of these findings for the mechanism driving the MIT in bulk RENiO₃.