Zinc-carbon bonds bond energies

Thermochemical attention in this chapter is directed towards compounds with carbon—zinc bonds, i.e. species that are usually labeled organometallic. The thermodynamic properties that we discuss are restricted to the enthalpy of formation (often called the heat of formation ), enthalpy of vaporization and carbon—zinc bond energies. We forego discussion of other thermochemical properties such as entropy, heat capacity or excess enthalpy. The energy units are kJmoU where 4.184 kJ is defined to equal 1 kcal. 

The various carbon-metal bond energies needed have been taken from the compilation of Skinner4, as have also the heats of vaporisation of the organometallic compounds. The heats of solution of liquid dimethylzinc and dimethylmercury have both been assumed to be +2 kcal.mole-1 (the heat of solution of dimethyl-zinc in water, to give dimethylzinc in aqueous solution, is, of course, a purely formal value). 

When the guest used is p-nitrophenylcholine carbonate (PNPCC) the Lewis acid zinc(n) activates the well-positioned carbonyl group in the P PCC Zn-cavitand towards reactions with external nucleophiles. The energy minimized structure of the PNPCC Zn-cavitand complex shows that cation-n interactions and C —O -Zn coordination bond occurs simultaneously. 

As with carbonic anhydrase the metal is in a cleft that exposes the active site. Interestingly the metal-free enzyme is inactive but the cobalt and nickel analogues are more active. It appears that the transition state complex, where the terminal amino acid side chain is held in place while the peptide bond is hydrolysed, requires six-fold co-ordination. The activation energy required to change from the tetrahedral to octahedral geometries is higher for zinc than the other metals. 

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