A series of homoleptic mononuclear 8-coordinate FeII and CoII compounds, [FeII(L²)₂](ClO₄)₂ (2), [FeII(L³)₂](ClO₄)₂ (3), [FeII(L⁴)₂](ClO₄)₂ (4), [CoII(L¹)₂](ClO₄)₂ (5), [CoII(L²)₂](ClO₄)₂ (6), [CoII(L³)₂](ClO₄)₂ (7), and [CoII(L⁴)₂](ClO₄)₂ (8) (L¹ and L² are 2,9-dialkylcarboxylate-1,10-phenanthroline ligands; L³ and L⁴ are 6,6'-dialkylcarboxylate-2,2'-bipyridine ligands), have been obtained, and their crystal structures have been determined by X-ray crystallography. The metal center in all of these compounds has an oversaturated coordination number of 8, which is completed by two neutral homoleptic tetradentate ligands and is unconventional in 3d-metal compounds. These compounds are further characterized by electronic spectroscopy, cyclic voltammetry (CV), and magnetic measurements. CV measurements of these complexes in MeCN solution exhibit rich redox properties. Magnetic measurements on these compounds demonstrate that the observed single-ion magnetic (SIM) behavior in the previously reported [FeII(L¹)₂](ClO₄)₂ (1) is not a contingent case, since all of the 8-coordinate compounds 2–8 exhibit interesting slow magnetic relaxation under applied direct current fields. Copyright © 2018 American Chemical Society.