With an objective to identify the factors in6uencing the stabilization of the unusual valence state Ni Ψ in perovskite-related oxides, the reduction of the Ruddlesden+Popper series precursors A nΨ1 Ni n O 3nΨ1 (A β«Ψβ¬ lanthanide and alkaline earth and n β«Ψβ¬ 1, 2, and 3) containing the Ni 2Ψ/ 3Ψ or Ni 3Ψ/ 4Ψ couple has been investigated systematically. La 2Ψx Sr x NiO 3Ψ , Ln 3 Ni 2 O 6Ψ (Ln β«Ψβ¬ lanthanide), and Ln 4 Ni 3 O 8 oxides containing the Ni Ψ/ 2Ψ couple have been obtained by a reduction of, respectively, La 2Ψx Sr x NiO 4 , Ln 3 Ni 2 O 7 , and Ln 4 Ni 3 O 10 with dilute hydrogen over a narrow range of temperature. During the reduction process, oxygen atoms are removed from the NiO 2 planes of La 2Ψx Sr x NiO 4 to give orthorhombic La 2Ψx Sr x NiO 3Ψ , but from the rock salt planes present in between the NiO 2 planes of Ln 3 Ni 2 O 7 and Ln 4 Ni 3 O 10 to give tetragonal Ln 3 Ni 2 O 6Ψ and Ln 4 Ni 3 O 8 , respectively. Bond length matching between the stretched Ni Ψ
+O and A+O bonds to minimize the internal stresses and coordination preference and size of the A cations are found to play an important role in accessing Ni Ψ . Although the Ni Ψ/2Ψ couple is isoelectronic with the Cu 2Ψ/ 3Ψ couple in copper oxide superconductors, the Ni Ψ/2Ψ oxides exhibit localized semiconducting properties due to a larger charge transfer gap.