Ligand Substitution in Ni(CO) ₄ ─Computational Discovery of a Long-Suspected “Dark Pathway” and a Detailed Analysis of the Intrinsic Reaction Coordinate

The reaction mechanism for carbon monoxide self-exchange in nickel tetracarbonyl and the substitution of CO by various incoming ligands was investigated in depth using DFT calculations. Early experimental work from the 1960s suggested that ligand substitution in Ni(CO)₄ proceeds via a unimolecular dissociative (<i>D</i>) mechanism. Although a bimolecular interchange (I-type) pathway had been proposed, definitive evidence for such a mechanism is still lacking. Our calculations now provide convincing support for an interchange route: all transition states (TSs) for the one-step interchange process were located and validated by vibrational analysis. The ligands studied─hydrogen cyanide, pyridine, trimethylamine, trimethylphosphine, triphenylphosphane, and trimethylarsine─show reaction barriers below 20 kcal/mol, largely independent of the functional used (B3LYP, B3PW91, or ωB97XD with 6-311+G(d,p)). Despite the bimolecular nature of the reaction and the diversity of donor atoms, the activation barriers proved surprisingly insensitive to ligand identity. To uncover more subtle ligand-dependent effects, additional B3LYP/6-311+G(d,p) calculations tracked topological and conceptual DFT indices along the full-reaction coordinates; for pyridine, NICS aromaticity indices were also followed. These analyses reveal nuanced differences among the ligands and suggest that the interchange pathway, though hidden under typical conditions, could become dominant with appropriate ligands and high concentrations.