The search for new compounds containing low valent metal atoms seems to be especially interesting since unusual chemical and physical properties are mostly related to the open-shell configurations of individual elements. Low valent In fluorides and oxides are mostly unknown and all experiments to reduce InF3 or In2O3, which date back into the thirties, failed. The reduction of InF3 with elemental In in the presence of Pt powder resulted in a new class of low valent fluorides such as PtIn7F13, Pt3In22F40 Pt2In14Ga3O8F15, in which characteristic building units are [PtIn6]10+ octahedra with very short Pt-In distances of 255 pm. Electronic structure calculations show, that within the PtIn6 octahedra strong Pt-In interactions are present. The extension of this field to transition metal In-Oxides has led to a whole series of filled and defect pentlandite type variants, e.g. TIn6(MO4)2 (T = Pt, Ir; Ru, Fe; M = Zn, Ga, Ge, In, Fe) with similar highly positive 18-electron [TIn6]n+ ions. It seems reasonable to assign the oxidation state +2 to In and thus negative oxidations states to the transition elements in these octahedral units. Especially the chemical stability of the low valent In oxides and fluorides against moisture or weak acids makes them promising candidates for new materials with unusual physical properties.
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