Group 2 and Group 12 We will discuss groups 2 an 12 together due to their similar reactivity. Group 12 has completely filled and low-lying d-orbitals, mimicking group 2. Magnesium compounds are the most important with respect to reactivity. Like the heavier alkali metals, the heavier alkali earth metals (Ca, Sr, Ba) are not as useful magnesium compounds, due to their tendency to be ionic rather than covalent compounds.

Beryllium Beryllium’s extremely high charge density gives some structural anomalies. No compelling reason to use due to toxicity.

Organoberyllium Synthesis More electronegative than Li and Mg so metathesis reaction with organolithium or Grignard works. BeCl LiPh  2 LiCl + BePh 2 Transmetallation with HgR 2 also works. HgR 2 + Be  R 2 Be + Hg

Organomagnesium Compounds The most important of these, RMgX, are the Grignard reagents. They were first isolated by Victor Grignard. Grignard reagents are mainly made by direct reaction of haloalkane with Mg in ether solvents (trace of I 2 ) CH 3 Br + Mg  CH 3 MgBr (ether, 20°C) Can also be made by transmetallation with organomercury: MgCl 2 + (CH 3 CH 2 ) 2 Hg  2 CH 3 CH 2 MgCl + HgCl 2

Organoberyllium Structures Compounds are electron-deficient with a high charge density on Be 2 + and thus display some uncommon bonding arrangements. BeMe 2 is a polymeric material, with the 3-center, 2-electron (3c,2e) bond

Structures BeMe 2 Dimer with Coordinating Solvent (L) Coordinating solvents break the polymeric structure down to dimers Be, Mg covalent : tendency to adopt a four coordinate tetrahedral structure via bridging atoms. However, steric effects can lead to decreased association. Monomeric Be( t Bu) 2 and Mg{C(SiMe 3 ) 3 } 2

Beryllocene An unusual case of bonding exists in beryllocene (Cp 2 Be), due to the very small size of the beryllium center. (  1 -Cp(  5 -Cp)Be in the solid but electron diffraction and spectroscopy suggest a different structure. This structure is fluxional in the NMR, with all protons equivalent down to 163K

Schlenk Equilibrium In solution, Grignard reagents undergo a complex equilibrium called the Schlenk equilibrium. This equilibrium is important in the reactivity of Grignard reagents:

Schlenk Equilibrium This equilibrium is more pronounced, and thus a Grignard is more reactive, as: the halide becomes less basic more electronegative (up the group) the R group becomes a more stable carbanion greater branching Grignard reagents exist as a mixture of species due to equillibria in solution – 2RMgX == MgR2 + MgX2 and more complex processes

Magnesium Dialkyls Magnesium dialkyls can be synthesized by transmetallation. Mg + Et 2 Hg  MgEt 2 + Hg (THF) Taking advantage of the Schlenk equilibrium. Addition of a very strong donor ligand (e.g. dioxane) will precipitate the dihalomagnesium adduct, leaving the dialkyl in solution:

Structure of the Grignards The structures of magnesium mono- and di-alkyl compounds are similar to that of beryllium. They range in oligomerization from polymers to monomers depending on the R group and presence of base. Halide bridges with 2c,2e bonds preferred over alkyl bridges. Polymeric species are common in weak donor solvents (Et 2 O) and/or with high Grignard concentration.

Crystal Structures Crystallization commonly carries coordinated solvents because Grignard reagents are usually not soluble in hydrocarbons. Oligomerization of both RMgX and R 2 Mg compounds is a function of steric bulk and coordinative saturation. The larger the group, the less oligomerization The stronger the base, the less oligomerization A polymer A monomer, due to steric bulk

Crystal Structures Structure of Cp 2 Mg contrast with Be

An Unusual Grignard This Grignard has no M-C bond: Organometallics, 1997, 16(4), 503.

Grignard Reagents Particularly useful carbanion reagents in organic chemistry Presences of ether can some limite to uses – can form unwanted complexes with Lewis acidic reagents (e.g. BX 3 ) (can be avoided by using lithium reagents that are hydrocarbon soluble)

Group 12 Synthesis and Structure Zinc organometallics are most commonly prepared by metathesis with alkyllithium or alkylaluminum. ZnCl RLi  R 2 Hg + 2 LiCl 3 Zn(OAc) AlR 3  3 ZnR Al(OAc) 3 Note that in the case of Al reagents that this does not conform to the electronegativity (1.61 Al, 1.65 Zn) but does correlate with hardness (Zn 2+ softer with CH 3 - and Al 3+ with Cl - ) Transmetallation: Zn + HgR 2  Hg + ZnR 2 Notably, group 12 compounds are not a very electropositive metal centres. They are much less reactive than the groups 1 and 2, and thus have seen less utility in synthesis.

Organozinc Compounds Reactivity – pyrophoric and readily protonated/hydrolyzed Mild Lewis acidity – will coordinate amines esp. if chelating Do not behave as good Lewis acids. Lewis acidity decreases on going down the group Carbanionic character exhibited by reactivity with ketones to yield alkoxide – same as Li, Al, Mg (note this does not go with R 2 Hg or Cd) Decreasing bond polarity and carbanion like behavior on going down the group – e.g. Dialkylmercury compounds do not add across ketones On going down the group the softness of the metal center increases – Organomercury compounds show a strong tendency to bind to sulfur leading to high toxicity

Structures of Organozinc Compounds Zinc shows the kind of diversity that is common among the less electropositive metals, although it favours the linear arrangement.

Structures of Organozinc Compounds Generally formation of linear species that are not associated in solid, liquid, gas or haydrocarbon solution Suggests that these species possess 2c, 2e bonds Do not complete octect through association via alkyl bridges like Mg and Be

Structure and Zinc Cyclopentadienyl compounds of Zn – monomeric pentahapto in the gas phase zig zag pentahapto to two Zn polymer in the solid state