Ziegler is perhaps best remembered for his work with Giulio Natta on
what are called Ziegler-Natta catalysts. These catalysts are
typically based on titanium compounds and organometallic
aluminium compounds, such triethylaluminium,
(C2H5)3Al and are used to
polymerize terminal 1-alkenes.
n CH2=CHR → -[CH2-CHR]n-
Together they won the Nobel Prize in Chemistry in 1963.
The organometallic compounds to be considered in this course are those containing a M-C bond, excluding carbonyls (M-CO), cyanides (M-CN) or carbides (M-C). A useful subdivision is by the type of M-C bond:
Ionic organometallic compounds are generally formed from elements such as sodium, potassium etc. where the metals are considered electropositive. If the organic groups are able to delocalise the negative charge over several carbon atoms then less electropositive elements like magnesium can also form ionic compounds, eg Cp2Mg. In this case the charge is considered to be delocalised over each of the five carbon atoms in each ring.Covalent
The simplest model of the M-C bond is where it consists of essentially a single covalent 2-electron bond. These compounds are often volatile and are comparable to typical organic compounds being soluble in organic solvents.Electron deficient
Electron deficient organometallic compounds are generally associated with elements that have less than half-filled valence shells and are designated as such because of an insufficient number of valence electrons to allow all the atoms to be linked by traditional two-electron two-centre bonds. The compounds often have bridged or polymeric structures. The methyl derivatives of Li, Be and Al are found to be 3-D polymers, linear chains and dimeric respectively and despite the increase in RMM of the monomeric unit there is actually an increase in volatility.
|Compound||RMM of monomeric unit||Structure||Volatility|
|BeMe2||39.08||Linear chain||sublimes at 473 K|
|AlMe3||72.08||Dimer||Melts at 288 K|
there is no such thing as electron deficient compounds, only theory deficient chemists.Stability of Organometallic compounds
The M-C bond energies for some methyl derivatives are shown in the Table below. Plotting these values against Atomic Number of the metal shows that there is a decrease down a Group. This behaviour is expected since there should be better orbital overlap between similar valence orbitals and this would decrease for the larger more diffuse elements lower down a Group.
|Me2M||D / kJ mol-1||ΔHf / kJ mol-1||BP /K||Me3M||D / kJ mol-1||ΔHf / kJ mol-1||BP /K||Me4M||D / kJ mol-1||ΔHf / kJ mol-1||BP /K|
In general terms thermodynamic stability means that the ΔG° is negative i.e. the energy of the products is more stable than that of the starting materials. Since little free energy data is available, it is often assumed that ΔH can be considered as a guide remembering that the entropies of gases are much larger than for liquids, which is again much larger than for solids and this can be taken into account as well.
if we take as an example the thermal decomposition of
EtLi → LiH + CH2=CH2
ΔHf EtLi = -58.55 kJ mol-1
ΔHf CH2=CH2 = +52.40 kJ mol-1
ΔHf LiH = -90.45 kJ mol-1
so that the overall enthalpy change is: = (RHS - LHS) = 52.40 - 90.45 + 58.55
= +20.50 kJ mol-1
This therefore suggests that the data favours the stability of EtLi over the products. However, given that at room temperature the Entropy of gaseous ethylene (ethene) is high TΔS = +64.4 kJ mol-1 and the entropies for solids will be much smaller, then using ΔG = ΔH - TΔS it is likely that ΔG will be a sizable NEGATIVE value which would suggest that EtLi should be unstable.
Calculations of free energy would suggest that many organometallic compounds should be unstable. However, kinetic stability needs to be considered as well since if there is no low activation energy pathway for a reaction to proceed then it may be very slow.Stability to Oxidation
All organometallic compounds are expected to be thermodynamically unstable with respect to oxidation to give metal oxide, carbon dioxide and water. Some are spectacularly so, being highly pyrophoric. In general organometallic compounds need to be handled under dry nitrogen or some other inert gas to avoid oxidation.Stability to Hydrolysis
Hydrolysis of organometallic compounds often involves nucleophilic attack by water which is accentuated when there are low-lying empty orbitals on the metal atom. This is seen for Groups I, II and for Zn, Cd, Al, Ga etc and the speed of hydrolysis is dependent on the M-C bond polarity. For "Me3Al" rapid attack occurs whereas Me3B is unaffected at room temperature.
The nomenclature used to denote the idea that the 5-carbons of the cp ring are equally attached is by the use of η5. For the case where only 1-carbon is attached then the designation would be η1. Here the Greek letter "eta" is used and the term is generally called "hapticity".
Sodium and potassium form intensely coloured salts with aromatic compounds.
The alkali metal is oxidised and transfers one electron to the aromatic
system and this becomes a paramagnetic radical anion:
Na + naphthalene → Na+[C8H10]-
In the case of napthalene, the salt is deep blue.