Spectator group

Ligands are not always innocent spectator groups. Cationic j -diiminate alkyl complexes of aluminium, rather than undergoing insertion in the Al-alkyl bond, show attack of the unsaturated substrate (ethene or acetylene) at the ligand (Scheme 1). Surprisingly, the C-C coupling reaction observed with ethene is reversible [42]. 

Based on the above arguments, we expect reaction (4) to be much more favorable than reaction (3) because (3) involves attack on a stronger bond. However, there is a second equally important factor involved in the difference in reaction enthalpies for (3) and (4). The extra Mo=0 bond of 2 would appear to be a spectator to reaction (4), but in fact it helps promote the reaction. The reason is that in the product, 5, this spectator group is free to utilize two Mo d7T orbitals to form a super double bond, whereas in the reactant, 2, the second Mo=0 bond (the one involved directly in the reaction) requires one of these drr orbitals. Thus the spectator Mo=0 bond changes from a double bond to a super double bond when the aUyl reacts with the other Mo=0 bond. The net result then is that reaction (4) is more favored than (3) by 33 kcal. 

Mora, J. R., Kirby, A. J., Nome, F. (2012). Theoretical Study of the Importance of the Spectator Groups on the Hydrolysis of Phosphate Triesters. The Journal of Organic Chemistry, 77, 7061-7070. 

We have adopted a similar strategy to introduce redox active spectator groups onto tpy ligands. A typical example is shown in Figure 4, in which the synthesis of Fctpy, which bears a ferrocenyl substituent is described. The new ligand shows an Fc/Fc+ process at +0.12 V [7]. 

Key words Generalized product function - Group function - Ab initio model potentials - Embedding -Spectator groups... 

The high rate of spontaneous hydrolysis of tris-2-pyridyl phosphate (96) to the corresponding dietser (97) was explained by the activating elfects of the non-leaving ( spectator ) groups on P-OAr cleavage, and not by intramolecular catalysis as discovered by Kirby and co-workers (Scheme 25). ... 

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