Four- and Five-Coordinate Osmium(IV) Nitrides and Imides: Circumventing the “Nitrido Wall”

Osmium nitride chemistry is dominated by osmium(VI) in octahedral or square-pyramidal coordination. The stability of the d2 configuration and preference of the strong σ- and π-donor nitride for apical coordination is in line with the Gray–Ballhausen bonding model. In contrast, low-valent osmium(IV) or other d⁴ nitrides are rare and have only been reported with lower coordination numbers (CN ≤ 4), thereby avoiding π-bonding conflicts of the nitride ligand with the electron-rich metal center. We here report the synthesis of the square-planar osmium(IV) nitride [Os^IVN(PNP)] (PNP = N(CHCHPtBu₂)₂). From there, a square-pyramidal isonitrile adduct could be isolated, which surprisingly features basal nitride coordination. Analysis of this five-coordinate d⁴ nitride shows an unusual binding mode of the isonitrile ligand, which explains the preference of the weakest σ-donor and strongest π-acceptor isonitrile for apical coordination.