Influence of the d Orbital Occupation on the Structures and Sequential Binding Energies of Pyridine to the Late First-Row Divalent Transition Metal Cations: A DFT Study

The ground-state structures and sequential binding energies of the late first-row divalent transition metal cations to pyridine (Pyr) are determined using density functional theory (DFT) methods. Five late first-row transition metal cations in their +2 oxidation states are examined including: Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺. Calculations at B3LYP, BHandHLYP, and M06 levels of theory using 6-31G* and 6-311+G­(2d,2p) basis sets are employed to determine the structures and theoretical estimates for the sequential binding energies of the M²⁺(Pyr)_x complexes, where x = 1–6, respectively. Structures of the Ca²⁺(Pyr)_x complexes are compared to those for the M²⁺(Pyr)_x complexes of Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ to further assess the effects of the d-orbital occupation on the preferred binding geometries. The B3LYP, BHandHLYP, and M06 levels of theory yield very similar geometries for the analogous M²⁺(Pyr)_x complexes. The overall trends in the sequential BDEs for all five metal cations at all three levels of theory examined are highly parallel, and are determined by a balance of the effects of the valence electronic configuration and hybridization of the metal cation, but are also influenced by repulsive ligand–ligand interactions. Present results for the M²⁺(Pyr)_x complexes are compared to the analogous complexes of the late first-row monovalent transition metal cations, Co⁺, Ni⁺, Cu⁺, and Zn⁺ previously investigated to assess the effect of the charge/oxidation state on the structures and sequential binding energies. Trends in the sequential binding energies of the M²⁺(Pyr)_x complexes are also compared to the analogous M²⁺(water)_x, M²⁺(imidazole)_x, M²⁺(2,2′-bipyridine)_x, and M²⁺(1,10-phenanthroline)_x complexes.