Alkyl-Substitution Effect of the Diamine on Coordination Geometry and Ligand-Field Strength of Nickel(II) Mixed-Ligand Complexes Containing N-Alkylethylenediamine, Acetylacetonate, and Nitrate Ligands

Abstract

Abstract Structures and spectral properties have been investigated for nickel(II) mixed-ligand complexes, [Ni(Me4en)(acac)(NO3)] (1), [Ni(EtMe3en)(acac)(NO3)] (2), [Ni(asym-Et2Me2en)(acac)(NO3)] (3), [Ni(Et3Meen)(acac)(NO3)] (4), and [Ni(Et4en)(acac)(NO3)] (5) (Me4en = N,N,N′,N′-tetramethylethylenediamine, EtMe3en = N-ethyl-N,N′,N′-trimethylethylenediamine, asym-Et2Me2en = N,N-diethyl-N′,N′-dimethylethylenediamine, Et3Meen = N,N,N′-triethyl-N′-methylethylenediamine, Et4en = N,N,N′,N′-tetraethylethylenediamine, acac = acetylacetonate). The crystal structures of complexes 2 and 3 have been determined. These complexes have 6-coordinated octahedral (Oh) structure with a bidentate nitrate in the solid state as well as in 1,2-dichloroethane or acetone solution, while the nitrate partially dissociates in nitromethane to establish an equilibrium with square-planar (Sp) form [Ni(acac)(diam)]+ where diam represents a diamine. The degree of dissociation of NO3− increases in the order of diam: Me4en < EtMe3en < asym-Et2Me2en < Et3Meen < Et4en, indicating that the bulkier diamine more favors the Sp form. This relative stabilization of the Sp form can be attributed to relief of the steric strain in the Oh species. Effects of bulky substituents in the diamine on the ligand-field strength are also discussed.