Syntheses, Structures, Spectroscopic Properties, and Thermal Behavior of Nickel(II) Mixed-Ligand Complexes with <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-Tetramethylethylenediamine, Benzoylacetonate, and a Halide Anion

Abstract A series of nickel(II) complexes comprising N,N,N′,N′-tetramethylethylenediamine (tmen), benzoylacetonate (bzac), and a halide anion (X), Ni(tmen)(bzac)X·n(H2O)(n = 1–4, X = Cl, Br, I), have been synthesized. A crystallographic study revealed that the chloride anion coordinates to the central metal, while bromide and iodide anions are located in the outer coordination sphere as the counter anion; the metal has a slightly distorted octahedral geometry, and the two monodentate ligands (aqua and chloro) are in the cis-position. In bromide and iodide complexes, having two water molecules of crystallization, a ladder-like hydrogen bond network is formed among halide anions, aqua ligands, and water of crystallization. In a chloride complex having no water of crystallization, a dimer is formed by strong intermolecular hydrogen bonding between the aqua ligand and the chloro and the benzoylacetonate ligands of the neighboring molecule. The behaviors of the complexes in solution vary, depending on the nature of the solvents, including deaquation and disproportionation, causing contrasting color changes of the solutions. The presence of a proposed intermediate in the disproportionation reaction, namely a 5-coordinated complex, was confirmed. Thermal gravimetric analysis suggests that these complexes lose water of crystallization, undergo deaquation–anation, and then lose the coordinated water before disproportionation into two ternary complexes.