Low valent magnesium compounds

A number of stable low valent magnesium compounds containing a metal-metal, Mg-Mg, bond, where magnesium exhibits the formal oxidation state of +1 are known. These compounds generally have the formula L₂Mg₂, where L represents a bulky ligand.^[1] The first examples of these stable magnesium(I) compounds were reported in 2007.^[2] The chemistry of Mg is dominated by the +2 oxidation state and prior to 2007 only examples of crystalline compounds with short Mg-Mg distances that may indicate an Mg-Mg bond were known, such as the ternary metal hydrides Mg₂RuH₄, Mg₃RuH₃, and Mg₄IrH₅ and magnesium diboride,^[3] Calculations had also indicated the stability of the Mg₂²⁺ cation.^[4]

The preparation of the first compounds made involved the reduction of Mg^II iodine complexes with potassium metal and the bulky ligands were:^[2]

• a guanidinate, "priso", [(Ar)NC(NPrⁱ₂)N(Ar)]⁻ where Ar = 2,6-diisopropylphenyl and Prⁱ = iso-propyl
• a ketiminate, "nacnac", {[(Ar)NC(Me)]₂CH}⁻,- where Ar = 2,6-diisopropylphenyl and Me = methyl

Both examples have the formula L₂Mg₂, where L represents the bulky anionic bidentate ligand.^[2] X-ray crystallographic studies showed an Mg-Mg bond length of 285.1 pm and 284.6 pm.^[2] Theoretical studies indicate an essentially ionic formulation Mg₂²⁺(L⁻)₂.^[2] The Mg₂²⁺ ion is the group 2 analogue of the group 12 Hg₂²⁺ (present in e.g. mercury(I) chloride) and Cd₂²⁺ ions (present in cadmium(I) tetrachloroaluminate).

Since then a variety of stable Mg(I) compounds have been prepared, some melting over 200 °C, some colorless, others colored, but all involving very bulky ligands.^[1] Also complexes of the LMgMgL with monodentate ligands have been prepared and in these the coordination of the Mg atom increases from three to four.^[1] The magnesium(I) dimers have proved to be useful reducing agents, for example in the preparation of tin(I) compounds.^[5]