Ligand Substitution in Ni(CO) <sub>4</sub> ─Computational Discovery of a Long-Suspected “Dark Pathway” and a Detailed Analysis of the Intrinsic Reaction Coordinate
Bibliographic record
Abstract
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 ( D ) 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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".