Theoretical Investigation of the Coordination of N₂ Ligands to the Cluster Ni₃

Abstract

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]:[Ni3], 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 N2 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.

Impurity Atoms on Small Transition Metal Clusters. Insights from Density Functional Model Studies

“…We also studied the dissociative adsorption of up to 6 H 2 molecules on different metal tetramers [43,44,48]. In addition, we considered molecular adsorption of one or more CO or N 2 molecules on selected nickel clusters [45,50]. The obtained results indeed showed that hydrogen impurity atoms on the clusters cause an extension of the interatomic distances in the metal moieties, bringing them close to the experimental values.…”

“…In particular, we modeled clusters containing up to 6 nickel atoms [35][36][37]45] as well as tetra-atomic clusters of palladium [39], iridium [33,40,41,47,48], and platinum [34,42], and hexa-atomic rhodium [38]. All these clusters were considered to interact with hydrogen atoms, while on some clusters we also modeled the coordination of C and O.…”

“…We modeled nondissociative adsorption of three to nine N 2 molecules on neutral and charged Ni 3 clusters [45]. Although N 2 molecules are often considered as inert ligands, their interaction with a Ni 3 cluster was found to be quite strong, with a BE per ligand above 100 kJ/mol for the most stable structures featuring on-top coordinated N 2 or combinations of on-top and bridge bound ligands.…”

“…Here, computational modeling can make important contributions. Stimulated by the success of Gates and co-workers in preparing and exploring such cluster systems, we joined the efforts to understand the behavior of (small) metal clusters, by applying density functional methods [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. These methods have become essential for investigating various chemical systems, in particular transition metal clusters [51][52][53][54][55][56][57][58][59][60].…”

“…The obtained structures were checked for a possible JahnTeller effect by inspecting the orbital occupation and/or, where appropriate, by decreasing the symmetry. In most of the studies symmetry constraints were initially applied to locate minima; the stability of the resulting structures was checked by calculating vibrational frequencies without symmetry constraints, except for Ni 3 (N 2 ) n [45]. Since clusters of 4-6 metal atoms only exhibit a rather limited number of topologies, we refrained from using elaborate techniques that identify structures of minimum energy, but searched for stable structures simply by checking various possible initial geometries.…”