Recently synthesized Rh-Ni trinuclear complexes hexacoordinated with sulfur ions, 3-aminopropanethiolate (apt) metalloligand [Ni{Rh(apt)$_{3}$}$_{2}$](NO$_{3}$)$_{n}$ ($n$ = 2, 3, 4), are found to be chemically interconvertible between the nominal Ni$^{2+}$ and Ni$^{4+}$ states. In order to clarify the origins of their interconvertible nature and the stability of such a high oxidation state as the tetravalency from the physical point of view, we have systematically investigated the local 3$d$ electronic structures of [Ni{Rh(apt)$_{3}$}$_{2}$](NO$_{3}$)$_{n}$ by means of soft X-ray core-level absorption spectroscopy (XAS). The experimental data have been reproduced by the single-site configuration-interaction cluster-model simulations, which indicate that the charge-transferred configurations are more stable than the nominal $d$-electron-number configuration for $n=3,4$ leading to the prominent charge-transfer effects. These are also supported by S $K$-edge XAS of [Ni{Rh(apt)$_{3}$}$_{2}$](NO$_{3}$)$_{n}$. Our results imply that the found charge-transfer effects have a key role to realize the interconvertible nature as well as the stability of the high oxidization state of the Ni ions.
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