Dimethylglyoxime

Dimethylglyoxime is a chemical compound described by the formula CH3C(NOH)C(NOH)CH3. It is the dioxime derivative of the diketone diacetyl (also known as 2,3-butanedione). DmgH2 is used in the analysis of palladium or nickel. Its coordination complexes are of theoretical interest as models for enzymes and as catalysts. Many related ligands can be prepared from other diketones, e.g. benzil.

Preparation
Dimethylglyoxime can be prepared from methyl ethyl ketone first by reaction with ethylnitrite followed by conversion of the biacetyl monoxime using sodium hydroxylamine monosulfonate:
 * CH3C(O)CH2CH3 +  CH3CH2ONO  &rarr;  CH3C(O)C(NOH)CH3  +  CH3CH2OH
 * CH3C(O)C(NOH)CH3 +  HONHSO3Na  &rarr;  CH3C(NOH)C(NOH)CH3  +  NaHSO4

Ni(dmgH)2
As an analytical reagent, dmgH2 is often used as a solution in ethanol. It is the conjugate base, not dmgH2 itself, that forms the complexes. Furthermore, a pair of dmgH- ligands are joined through hydrogen bonds to give a macrocyclic ligand. The most famous complex is the bright red Ni(dmgH)2, formed by treatment of Ni(II) sources with dmgH2. This planar complex is very poorly soluble and is used for the gravimetric determination of nickel, e.g. in ores.

Cobaloximes
The nitrogen atoms in dmgH2 and its complexes are sp2 hybridized. Because of the planarity of the resulting ligand, the macrocycle [dmgH]22- resembles some biologically important macrocyclic ligands, as found for example in vitamin B12 and myoglobin. A well known family of model complexes, the cobaloximes, have the formula CoR(dmgH)2L, where R is an alkyl group and L is typically pyridine. In this complex, L and R occupy “axial” positions on the cobalt, perpendicular to the plane of the (dmgH)2.