Dual Electrophilic–Nucleophilic Behavior of M(II) Centers (M = Co, Ni, Cu) in M(acac)2 Complexes: Insights into Intermolecular Noncovalent Interactions

Quantum-chemical calculations were performed to elucidate the electrophilic and nucleophilic properties of first-row transition-metal centers in M(acac)2 (M = Co2+, Ni2+, Cu2+; Hacac = acetylacetone), as well as the nature of the resultant M···N/I interactions with pyrazine and molecular iodine. The Co and Ni centers exhibit electrophilic–nucleophilic dualism: they can act as electrophiles to form semicoordination bonds, and also function as nucleophiles to form halogen bonds via their dz2 orbitals. By contrast, the Cu center behaves exclusively as an electrophile and yields stable π-hole complexes. The M···N and M···Ielectron belt bonds in M(acac)2···C4H4N2 and M(acac)2···I2(A) are semicoordinate interactions dominated by noncovalent character. The Iσ-hole···M metal-involving halogen bond within M(acac)2···I2(B) exhibits partial covalent character, while the bifurcated I···(O,O) halogen bonds in M(acac)2···I2(C) correspond to typical closed-shell noncovalent interactions. The primary stabilizing factors of these complexes are determined by the nature of the metal centers. London dispersion forces play a pivotal role in the nucleophilic complexes M(acac)2···I2(B). For the electrophilic complexes M(acac)2···C4H4N2/I2(A), the stability of Co-containing complexes is governed by electrostatic interactions, while Ni-containing complexes are stabilized by dispersion forces. Both electrostatic and dispersion forces contribute to stabilizing Cu-based complexes.