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 d(2) configuration and preference of the strong sigma- and pi-donor nitride for apical coordination is in line with the Gray-Ballhausen bonding model. In contrast, low-valent osmium(IV) or other d(4) nitrides are rare and have only been reported with lower coordination numbers (CN <= 4), thereby avoiding pi-bonding conflicts of the nitride ligand with the electron-rich metal center. We here report the synthesis of the square-planar osmium(IV) nitride [(OsN)-N-IV(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 d(4) nitride shows an unusual binding mode of the isonitrile ligand, which explains the preference of the weakest sigma-donor and strongest pi-acceptor isonitrile for apical coordination.