Addition to pi ligands

In organometallic chemistry, the Green–Davies–Mingos rules predict the regiochemistry for nucleophilic addition to 18-electron metal complexes containing multiple unsaturated ligands. In general, complexation enhances the susceptibility of unsaturated hydrocarbon toward nucleophilic attack. The rules were developed by Stephen G. Davies, Malcolm Green, and Michael Mingos in 1978

Nucleophilic attack is preferred on even-numbered polyenes (even hapticity).

Nucleophiles preferentially add to acyclic polyenes rather than cyclic polyenes.

Nucleophiles preferentially add to even-hapticity polyene ligands at a terminus. Nucleophiles add to odd-hapticity acyclic polyene ligands at a terminal position if the metal is highly electrophilic, otherwise they add at an internal site.

Simplified: even before odd and open before closed

The following is a diagram showing the reactivity trends of even/odd hapticity and open/closed pi-ligands.

The metal center is electron withdrawing. This effect is enhanced if the metal is also attached to a carbonyl. Electron poor metals do not back bond well to the carbonyl. The more electron withdrawing the metal is, the more triple bond character the CO ligand has. This gives the ligand a higher force constant. The resultant force constant found for a ligated carbonyl represents the same force constant for pi ligands if they replaced the CO ligand in the same complex.

Nucleophilic addition does not occur if kCO* (the effective force constant for the CO ligand) is below a threshold value

The following figure shows a ligated metal attached to a carbonyl group. This group has a partial positive charge and therefore is susceptible to nucleophilic attack. If the ligand represented by L_n were a pi-ligand, it would be activated toward nucleophilic attack as well.

Incoming nucleophilic attack happens at one of the termini of the pi-system in the figure below:

In this example the ring system can be thought of as analogous to butadiene. Following the Green–Davies–Mingos rules, since butadiene is an open pi-ligand of even hapticity, nucleophilic attack will occur at one of the terminal positions of the pi-system. This occurs because the LUMO of butadiene has larger lobes on the ends rather than the internal positions.

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