A series of osmium(VI) nitrido complexes containing Schiff base ligands, [Os-VI(N)(L)Cl] (L = salophen or salen), have been synthesized by reaction of the ligand with [NBu4II][Os-VI(N)Cl-4] in the presence of 2,6-dimethylpyridine. The v(Os equivalent to N) for the salophen complexes occur at around 1070 cm(-1) and are insensitive to the nature of the substituents present on the Schiff base ligand. The structures of [Os(N)(salophen)(MeOH)]ClO4 and [Os(N)(5,5-Cl(2)salophen)(MeOH)]ClO4 have been determined by X-ray crystallography, and the Os equivalent to N bond distances are 1.651 and 1.66 Angstrom, respectively. The osmium(VI) nitrido complexes react rapidly with triphenylphosphine to produce the corresponding osmium(IV) phosphoraniminato complexes, [Os-IV(NPPh3)(L)CI]. The osmium(IV) complexes exhibit reversible Os-V/IV and Os-IV/III couples in cyclic voltammetry. The E-1/2 values show linear correlations with the Hammett constants sigma(p) of the substituents on the Schiff base ligand. The structure of [Os-IV(NPPh3)(salophen)Cl] has been determined by X-ray crystallography. The rather long Os-N(P) bond length (1.92 Angstrom) and acute Os-N-P bond angle (149.6 degrees) suggest that there is no significant multiple-bond character in the Os-N bond. The kinetics of nitrogen atom transfer from a series of 5,5'-disubstituted salophen nitrido complexes to PPh3 have been studied in CH3CN at 25.0 degrees C by stopped-flow spectrophotometric method. The following rate law was obtained: -d[Os(VI)]/dt = k(2)[Os(VI)][PPh3]. The reactivities of the complexes were found to follow a Hammett correlation of log(k(X)/k(H)) with sigma(p), with a rho value of 1.9 +/- 0.1. The positive rho value is consistent with a transition state involving electrophilic attack by the nitrido ligand on the phosphorus atom.