Noncovalent Interactions of Ni⁺ with N-Donor Ligands (Pyridine, 4,4′-Dipyridyl, 2,2′-Dipyridyl, and 1,10-Phenanthroline): Collision-Induced Dissociation and Theoretical Studies

Kinetic-energy-dependent collision-induced dissociation (CID) of complexes of a variety of N-donor ligands (N-L) with Ni⁺, Ni⁺(N-L)_x, is studied using guided ion beam tandem mass spectrometry. The N-donor ligands investigated include: pyridine, 4,4′-dipyridyl, 2,2′-dipyridyl, and 1,10-phenanthroline. For most of the Ni⁺(N-L)_x complexes, CID results in endothermic loss of a single neutral N-L ligand as the primary dissociation pathway. Sequential dissociation of additional N-L ligands is observed at elevated energies for the pyridine and 4,4′-dipyridyl complexes containing more than one ligand. The cross-section thresholds for the primary dissociation pathways are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) of the Ni⁺(N-L)_x complexes after accounting for the effects of multiple ion−neutral collisions, the kinetic and internal energy distributions of the reactants, and their lifetimes for dissociation. Density functional theory calculations at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* level are performed to obtain model structures, molecular parameters, and energetics for the neutral N-L ligands and the Ni⁺(N-L)_x complexes. In general, theory is found to overestimate the strength of binding to the first N-L ligand, and underestimate the strength of binding to additional ligands. Trends in the sequential BDEs of the Ni⁺(N-L)_x complexes are examined and compared to complexes of Ni⁺, to several other ligands previously investigated. The trends in the sequential BDEs are primarily determined by the valence electronic configuration and the effects of sd-hybridization of Ni⁺ but are also influenced by repulsive ligand−ligand interactions. Natural bond orbital analyses indicate that the binding in these complexes is primarily noncovalent.