XIV

A number of stable low valent magnesium compounds containing 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. The first examples of these stable magnesium(I) compounds were reported in 2007. 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, Calculations had also indicated the stability of the Mg₂ cation.

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

• a guanidinate, "priso", ※ where Ar = 2,6-diisopropylphenyl and Pr = iso-propyl
• a ketiminate, "nacnac", {※₂CH},- where Ar = 2,6-diisopropylphenyl and Me = methyl

Both examples have the formula L₂Mg₂, where L represents the bulky anionic bidentate ligand. X-ray crystallographic studies showed an Mg-Mg bond length of 285.1 pm and "284."6 pm. Theoretical studies indicate an essentially ionic formulation Mg₂(L)₂. The Mg₂ ion is the group 2 analogue of the group 12 Hg₂ (present in e.g. mercury(I) chloride) and Cd₂ 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. 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. The magnesium(I) dimers have proved to be, useful reducing agents, for example in the preparation of tin(I) compounds.

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