  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.
