Quasi two-dimensional (2D) oxide-based honeycomb lattices have attracted great attention for displaying specific electronic instabilities, which give rise to unconventional bonding patterns and unexpected magnetic exchange couplings. The synthesis of AgRuO , another representative exhibiting unique structural properties, is reported here. The stacking sequence of the honeycomb layers (Ru O ) differs from analogous precedents; in particular, the intercalating silver atoms are shifted from the middle of the interspaces and cap the void octahedral sites of the (□Ru O ) slabs from both sides. This way, charge neutral, giant 2D "molecules" of Ag/Ru O /Ag result; a feature that significantly enhances the overall 2D character of AgRuO . Measurements of magnetization have revealed extremely strong magnetic exchange coupling to be present, surviving to a temperature as high as 673 K, which is the temperature of thermal decomposition. No indication for long-range magnetic order has, however, been observed. Theoretical analyses confirm the pronounced 2D character of the electronic system, and in particular reveal the inter-honeycomb layer coupling J to be distinctly weak.

Open-shell solids exhibit a plethora of intriguing physical phenomena that arise from a complex interplay of charge, spin, orbital, and spin-state degrees of freedom. Comprehending these phenomena is an indispensable prerequisite for developing improved functional materials. This type of understanding can be achieved, on the one hand, by experimental and theoretical investigations into known systems, or by synthesizing new solids displaying unprecedented structural and/or electronic features. β-Ag3 RuO4 may serve as such a model system because it possesses a remarkable anionic structure, consisting of tetrameric polyoxoanions (Ru4 O16 )(12-) , and is an embedded fragment of a 2D trigonal MO2 lattice. The notorious frustration of antiferromagnetic (AF) exchange couplings on such lattices is thus lifted, and instead strong AF occurs within the oligomeric anion, where only one exchange path remains frustrated among the relevant six. The strong magnetic anisotropy of the [Ru4 O16 ](12-) ion, and the effectively orbital nature of its net magnetic moment, implies that this anion may reveal the properties of a single-molecule magnet if well-diluted in a diamagnetic matrix.

Open-shell transition metal oxides are capable of developing a rich diversity of electronic phases. The specific features evolving crucially depend on an intricate interplay of various local and long-range electronic interactions. Recently, the 5d transition elements have come into sharp focus because for these elements spin-orbit coupling (SOC) and onsite Coulomb repulsion (U) are on a comparable energy scale. For Ir the t level associated to an octahedral crystal field (CF) is split by SOC, giving rise to a J = spin state and rendering respective oxides like Sr IrO as Mott insulators. Transferring this scenario to iridium(V) oxides would lead to a diamagnetic ground state, J =0. However, reported experimental results do not lend unambiguous support for such an electronic state. Theoretical explanations for the breakdown of the J=0 magnetic state suffer from conspicuous discrepancies. In an attempt to empirically contribute to resolving the puzzle, Bi NaIrO was synthesized in high purity by precipitation from homogeneous solution; it represents an iridium(V) oxide where long range band structure effects and magnetic superexchange are minimized, and the t degeneracy is lifted geometrically. We managed to reduce the strength of paramagnetic response, lending support to a J =0 ground state of Bi NaIrO , exhibiting van Vleck type behavior.

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