Theoretical investigation of the coordination of N₂ ligands to the cluster Ni₃

Various structures of complexes of the cluster Ni₃ with 3 to 12 N₂ ligands were modeled with a gradient-corrected density functional method. The stability of different types of bonding was considered and the most stable structures of Ni₃(N₂)_x complexes (x = 3-9, 12) were determined for neutral, cationic, and anionic systems. For the most stable structure of the neutral complex with three and six N₂ ligands, we calculated average ligand binding energies of 116 and 98 kJ/mol, respectively; the binding energy per ligand decreases with increasing number of ligand molecules. For canonical ensembles of mono- and trinuclear complexes with N₂ ligands at varying molar ratio k = [N₂]:[Ni ₃], our results suggest that, in agreement with experiment, the complex Ni₃(N₂)₆ is among the dominating species at saturation; yet, at sufficiently large molar ratios k, the trinuclear complex with seven ligands, not observed in experiment, also plays an important role in the simulated distributions. It is unclear whether this partial discrepancy in the product distribution originates from complications to simulate the experimental situation or from some aspects of the experimental procedure. Coordination of more than seven N₂ ligands is predicted to lead to a partial or full destruction of the Ni₃ moieties into mononuclear N₂ ligated complexes. The type of bonding of the N ₂ ligands (end-on, side-on, hapticity) was found to affect the characteristics of the complexes, e.g. the binding energy, the charge of the Ni₃ moiety, and the activation of the ligands. End-on coordination of N₂ molecules to a Ni atom of the Ni₃ unit entails the most stable type of bonding, whereas side-on coordination causes a stronger elongation of N-N bonds. The ionization potential and the electron affinity of a Ni₃ cluster were calculated to increase after association of ligands.