Group 2 organometallic chemistry

Overview

The organometallic chemistry of group 2 elements is relevant to organic chemistry, though in some cases, the interest is strictly academic.

Characteristics

In many ways the chemistry of group 2 elements (the alkaline earth metals) mimics that of group 12 elements because both groups have filled s shells for valence electrons. Thus, both groups have nominal valency 2. All group 2 elements are electropositive towards carbon and electronegativity decreases down the row. At the same time the atomic radius increases resulting in increasingly ionic character, higher coordination numbers, and increased ligand reactivity.

It is also important to assess the nature of the carbon-metal bond as it can just as easily be an ionic bond and not a covalent bond. For this reason metal cyanides (MCN), metal acetylides and carbides (for instance calcium carbide, an acetylene source) are excluded.

Many dialkyl group 2 metals are polymeric in the crystalline phase and resemble trimethylaluminium in three-center two-electron bond. In the gas-phase they are once again monomeric.

The metallocenes in this group are unusual. Bis(cyclopentadienyl)beryllium or beryllocene (CP₂Be) with a molecular dipole moment of 2.2 D rules out a classical metallocene with two hapticity 5 ligands. Instead the compound is a so-called slip ⁵η/¹η sandwich and on top of that also fluxional up to -125°C. While Magnesocene (Cp₂Mg) is a regular metallocene, bis(pentamethylcyclopentadienyl)calcium (Cp^*)₂Ca is actually bent with an angle of 147°. This angle increases going down the row.

Synthesis

Three important ways for synthesis of dialkyl and diaryl group 2 metal compounds is by metathesis:

MX₂ + R-Y → MR₂ + Y-X'

By transmetallation:

M'R₂ + M → MR₂ + M'

Manipulation of the Schlenk equilibrium of the organometal halides:

2 RMX + M → MR₂ + MX₂

Compounds

Organoberyllium

Organoberyllium chemistry is limited to academic research due to the cost and toxicity of beryllium, beryllium derivatives and reagents required for the introduction of beryllium, such as beryllium chloride.

See also: Berylliosis

Organomagnesium

Organomagnesium compounds are widespread. They are commonly found as Grignard reagents. The formation of alkyl or aryl magnesium halides from magnesium metal and an alkyl halide is attributed to a SET process. Relevant organic magnesium reagents outside the scope of Grignards are magnesium anthracene with magnesium forming a 1,4-bridge over the central hexagon used as a source of highly active magnesium and butadiene magnesium an adduct with butadiene and a source for the butadiene dianion.

Other group 2 organometallic

Further down this group calcium is nontoxic and cheap but organocalcium compounds are difficult to make. This is even more so for the remaining members strontium and barium. For the case of radium, only the gas-phase acetylide. One use for this type of compounds is in chemical vapor deposition.

See also

• Organometallic chemistry
• The chemistry of carbon bonded to other elements in the periodic table: