LAUSR

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 [OsIVN(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.