Density functional calculations on the conversion of azide and carbon monoxide to isocyanate and dinitrogen by a nickel to sulfur rebound mechanism

Density functional calculations (B3LYP & BP86) on a model system for the reaction between carbon monoxide and [Ni(N-3)('S-3')](-) ('S-3'(2-) = bis (2-mercaptophenyl) sulfide (2-)) predict a three-step mechanism. First, CO attacks the nickel to generate a pseudo "square-pyramidal" complex, in which CO, N-3(-), and two sulfides are basal and the central S atom of the 'S-3'(2-) ligand backs away from Ni to form a weak Ni-S apical bond. Then, CO inserts into the Ni-N bond and the weak apical Ni-S bond rebounds to its original strength as the nickel forms a square-planar intermediate. Finally, in a one-step process N-2 leaves as the remaining N atom and carbonyl rearrange to produce the nickel isocyanate product [Ni(NCO)('S-3')](-).