A new series of octahedral cobalt(II) complexes are formed when CoX2(X = Cl, Br, I, SCN) reacts with Hg(SCN)2 in the presence of Lewis bases. These complexes of stoichiometry CoHg(SCN)4•2L (L = THF, dioxane, pyridine, aniline) are pink to violet solids which slowly decompose to the blue crystalline solid, CoHg(SCN)4, the stable magnetic susceptibility standard. On further reaction of CoHg(SCN)4•2THF with mono-, bi-, and polydentate ligands in dry ethanol, complexes of the following types are obtained: CoHg(SCN)4•2L (L = PΦ3), CoHg(SCN)4•2LL (LL = trien), CoHg(SCN)4•3LL (LL = en, bipy), and CoHg(SCN)4•4LL (LL = phen). The stoichiometry of these were determined by elemental analysis. Possible structures of these are discussed with the help of mid and far infrared, visible, and ultraviolet spectroscopy, magnetic susceptibility, and X-ray powder diffraction. Some new i.r. bands like Co—P, Co—N, and Hg—S are assigned in the low region.