Main group organometallic chemistry [Mike Hill]
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organometallics
synthesis, structure and reactivity (i.e. s/p block) of compounds containing a metal to carbon bond metal (M) includes metalloids such as boron, silicon ligand (L) includes alkoxides (oxygen), thiolates (sulphur), enolates
electron precise
octet configuration at metal centre leads to molecular compounds with reduced lewis acidity/electrophilicity saturated centre -> inert
electron rich
leads to molecular compounds with reduced lewis acidity/electrophilicity
electron poor
too few valence e- provide pair for every bonded atom in a Lewis acidic/highly electrophilic molecule
how do organometallic compounds in groups 1,2 and 13 use its valence orbitals in bonding?
addition donors formation of multi centre bonds + aggregation
what happens to the polarity as the difference in electronegativity (effective nuclear charge) between metal and carbon increases
increases (+ reactivity of M-C bond increases)
effect of metal size on M-C bonding
larger metal = weaker M-C bond due to less efficient orbital overlap
which group are π-bonds restricted to?
1 heavier elements = σ-bonds only
kinetically labile
low energy decomposition energy highly reactive
how can kinetic lability be suppressed?
[increased thermal stability] via formation of Lewis base adduct or by use of ligands without hydrogens in the position (no β-hydrogen)
3 factors that govern the kinetic reactivity of main group organometallic with water and air
• high polarity of M-C bond (kinetic) • availability of low-lying empty orbitals at metal (kinetic) • availability of free electron pairs at metal (kinetic)
group 13 + air/water
vacant p orbital water/air sensitive
group 14 + air/water
electron precise water/air stable
group 15 + air/water
lone pair water/air sensitive
M-C bond formation - direct synthesis from metal and organic halide
2M + nR-X -> RnM + MXn or RnMXn useful for electropositive metas and alkyl magnesium = OXIDATIVE ADDITION driven by enthalpy of formation of M-X bond or MXn salt
M-C bond formation - redox transmetallation
M + RM’ -> RM + M’ metal of lower electronegativity (M) replaces metal of higher electronegativity (M’) M = electropositive group 1,2,13 metal M’ = Hg
M-C bond formation - metallisation (metal-hydrogen exchange)
R-M + R’-H -> R’-M + R-H driving force = relative acidities of the 2 C-H acids involved (RH and R’H) forward reaction will proceed if pKa of R’H is lower than RH
M-C bond formation - salt metathesis
R-M + M’-X -> R-M’ + MX important for generation of p-block M-C bonds driving force = formation of salt with high lattice enthalpy predicted by electronegativity of 2 metals involved - if M’ = more electronegative than M; reaction = feasible
M-C bond formation - addition reactions
[hydrometallation] common for B, Al, Si driving force = formation of C-H bond from weaker M-H bond [carbometallation] only occurs with v. electropositive metal
lithium + carbon - electronegativity
large difference BUT Li-C bonds = somewhat covalent due to polarising ability of Li+
LiR formation - direct synthesis
2Li + RX -> LiR + LiX high enthalpy of formation of LiX salt = exothermic reaction
LiR formation - transmetallation
2Li + HgR2 -> 2LiR + Hg based upon relative ease of reduction of Hg and Li
LiR formation - metal-halogen exchange
[at low temp.] Li-R + R’-X -> Li-R’ + R-X if carbanion formed = more stable, equilibrium will be driven to the right
Wurtz coupling process
R’Li + R’‘X -> R’R’’ + LiX