Monomeric Square-Planar Cobalt(II) Acetylacetonate: Mystery or Mistake?

No evidence was found for the existence of a previously reported mononuclear square-planar form of unsolvated cobalt(II) acetylacetonate, Co(acac)2, in all samples that have been obtained by using a variety of preparative techniques and crystallization conditions. It was confirmed that the structure of tetramer Co4(acac)8, reported back in 1964 by Cotton and Elder, is correct, the synthesis is reproducible, and the bulk material corresponds to single-crystal data. Additionally, the title compound can be isolated in tetranuclear form by reducing cobalt(III) acetylacetonate with cobalt metal in solvent-free conditions or by crystallization from a hexanes solution. At the same time, from noncoordinating halogenated solvents, Co(acac)2 crystallizes as a trinuclear core molecule, in which all Co atoms also exhibit octahedral coordination. From coordinating solvents such as ethanol, cobalt(II) acetylacetonate was found to appear in the form of its bis-adduct Co(acac)2(EtOH)2. On the basis of observations made in this work and the details presented in the original paper, we suggest that the reported mononuclear structure of square-planar acetylacetonate should likely contain copper instead of cobalt.