The use of tripodal N-Heterocyclic Carbene (NHC) ligands has enabled the synthesis of novel monomeric transition metal complexes with distinctive bonding motifs. Researchers have successfully developed isolable examples of midto high-valent, late transition metal imido species using these ligands in recent years. This concise review focuses on the contemporary synthetic approaches of terminal cobalt imido complexes that feature various nitrogen-anchored tripodal NHC ligands. The terminal cobalt imido complexes discussed in this review have exhibited several different reaction modes, including intramolecular migratory insertion, C–H bond insertion, and nucleophilic addition. Furthermore, singlecrystal X-ray diffraction analyses have revealed unique structural parameters of these complexes. To gain better insight into the electronic structures of these cobalt imido species, spectroscopic and computational studies have been conducted. These studies have provided evidence to support the cobalt imido speciesʼ imidyl radical or nitrene characters. Overall, the use of tripodal NHC ligands in the synthesis of terminal cobalt imido complexes has provided researchers with a powerful platform to explore and develop new late transition metal species with unique bonding motifs and reactivity patterns.

Metal-oxygen/water complexes such as metal-oxo, metal-hydroxo, metal-aqua, metal-peroxo, metal-hydroperoxo and metal-superoxo complexes play pivotal roles as reactive intermediates in various redox catalytic reactions including biological redox processes such as oxygen reduction in respiration and water oxidation in photosynthesis. This award article focuses on our recent progress in characterization of these metal-oxygen intermediates in terms of the structure, spectroscopic properties and redox reactivity. Ever since the X-ray crystal structure of a nonheme iron(IV)-oxo complex was reported, a number of metal-oxo, metal-hydroxo, metal-aqua, metal-peroxo, metal-hydroperoxo and metal-superoxo complexes were structurally characterized by X-ray crystallography. The reactivity of these metal-oxygen/water complexes was investigated extensively. Inspired by the pivotal role of calcium ion in the oxygen evolving complex in photosystem II, the effects of redox-inactive metal ions such as calcium ion on the redox reactivity of metal-oxygen/aqua complexes were studied together with binding of metal ions to metal-oxygen/aqua complexes. Metal-oxygen/water complexes exhibit electrophilic and nucleophilic reactivity depending on the metal, oxygen/water-derived ligand and supporting ligands. The electron-transfer reactivity of metal-oxygen/water complexes has been well evaluated in light of the Marcus theory of outer-sphere electron transfer.

A net-zero society needs integration of a large amount of renewable energy to a power grid while ensuring the quality of electricity. As the intermittent power from the renewables should be load-leveled using low-cost and high-power 1 MWh-scale stationary batteries, sodium-ion batteries that are comprised of abundant elements have attracted recent research interest. One of the most promising positive electrode materials for sodium-ion batteries is a family of Prussian blues (PB) and Prussian blue analogues (PBA), which delivers a large capacity of > 150 mAh/g at a high operating voltage of approximately 3.5 V vs. Na/Na⁺, meeting performance criteria for large-scale stationary batteries. However, for their commercialization, especially in Japan, further data accumulation including cycle stability, safety, and life cycle assessment is necessary. This review summarizes recent research and development of the PB and PBA electrodes for sodium-ion batteries.