In 1844, one of the first Chromium(II) compounds was discovered, Cr2(O2CCH3)4 (H2O)2, chromium(II) acetate hydrate4. Chromium(II) acetate hydrate is recognized as being somewhat anomalous, as mononuclear Chromium(II) are typically blue or purple and paramagnetic while chromium(II) acetate hydrate is red and diamagnetic1.
In chromium(II) acetate hydrate, chromium forms a dichromium (M-M bonded) complex, where chromium forms ÃÆ', Ã, and Î' bonds. The structure had been reported to be similar to that of Cu2(O2CCH3)4(H2O2) with a Cr-Cr bond distance of 2.641. Despite the compound being diamagnetic, such a large bond would not allow for substantial M-M bonding, implying the weakness of the ÃÆ', Ã, and Î' bonds2.
However, more recent studies have shown the reported distance only to be a rough estimate based upon the similar copper structures. A crystal structure study of the compound has shown the Cr-Cr distance to be 2.3623. The quadruple M-M interaction is shown to actually be a strong interaction, which has strong implications implying that M-M bonds may be important in the chemistry of metals of the first transition series as well as for the heavier metals3.
Chromium(II) acetate hydrate is a relatively stable chromium(II) compound, which rate of oxidation is relatively slow allowing less elaborate precautions for oxygen that other chromous preparations demand4. Each chromium(II) ion in the dimer has 4 3d electrons, but the complex is diamagnetic due to the formation of a quadruple bond between the two chromium(II) ions4.
Each chromium ion has six octahedral bonds. Four bonds are to oxygen atoms of the acetate groups, which act as a bridging ligands to the two chromium ions (see figure 1). One bond is to a water molecule and the sixth is to the other chromium ion4.
Experimental Section.
Preparation of Reaction Apparatus.
A Buchner flask is fitted with a dropping funnel using a two-holed rubber stopper.